Baptiste De

La Robertie

Yoann Pitarch
email: yoann.pitarch@irit.fr

Olivier Teste
email: olivier.teste@irit.fr

Measuring Article Quality in Wikipedia using the Collaboration Network

Collaboratively edited articles such as in Wikipedia suffer from well-identified problems regarding their quality, e.g., information accuracy, reputability of third-party sources, vandalism. Due to the huge number of articles and the intensive edit rate, the manual evaluation of article content quality is inconceivable. In this paper, we tackle the problem of automatically establishing the quality of Wikipedia articles. Evidences are shown to consider the interactions between authors and articles to assess the quality score. Collaborations between authors and reviewers are also considered to reinforce the discriminative process. This work gives a generic formulation of the Mutual Reinforcement principle held between articles quality and authors authority and take explicitly advantage of the co-edits graph generated by individuals. Experiments conducted on a set of representative data from Wikipedia show the effectiveness of our approach.

I. INTRODUCTION

Context. The free encyclopedia Wikipedia probably constitutes the most well-known collaborative system where any user can create and edit articles. Recent statistics 1 report almost 35 millions of articles in more than 280 languages, among which close to 2 and 5 millions of French and English articles respectively. This collaborative process, involved by more than 55 millions of contributors, generates 10 millions of edits each month, that is approximately 10 edits per second. This continuously increasing production of text data leads to various scientific challenges, among which the automatic quality assessment of the generated content.

The main strength of Wikipedia is to allow anyone to contribute to its content. Potential pitfall of such an open collaborative editing process is the emergence of doubtful or even radically poor quality contents, e.g., hoaxes, publicity, disinformation or acts of vandalism, that can be available for consultation for several weeks before being detected and corrected. A well-known example has concerned the involvement of the journalist John Seigenthaler in the Kennedy assassination, i.e., a fake content appeared in the wikipedia biography page of the former in 2005. The biography had also spread to the websites Answers.com and Reference.com and the erroneous content remained online for more than five months [START_REF] Cox | Truth in crowdsourcing[END_REF].

To overcome these limitations, various works tackle the problem of automatically assessing the quality of articles. Our work falls into this problem category. However, while substantial efforts have been made using explicit features such as length of the articles [START_REF] Blumenstock | Size matters: Word count as a measure of quality on wikipedia[END_REF] or implicit ones such as life cycles of texts [START_REF] Adler | A content-driven reputation system for the wikipedia[END_REF][4] [START_REF] Zeng | Mining revision history to assess trustworthiness of article fragments[END_REF], little attention [START_REF] Suzuki | Quality assessment of wikipedia articles using ¡i¿h¡/i¿index[END_REF] has been paid to indicators in the co-edit graph of authors. As motivated in the next section, we do think that considering some structural properties of the co-edit graph can significantly help in assessing the quality of Wikipedia articles.

Motivations. Besides the features considered by the state-ofthe-art approaches, we motivate the need for considering the co-edit graph in the quality score calculation by answering the following questions:

Are the co-edit graphs of top quality articles denser than of poor quality articles? To answer this question, we built the co-edit graph for each of these 2 categories 2 according to the following principles: editors having authored some contents in at least one article of the desired quality represent the set of vertices. It exists an undirected edge between two editors if they have co-edited at least an article. Edges are weighted by the number of articles the pair of author has co-edited normalized by the number of documents in the category. The answer to this question as well as others basic statistics on the graphs are shown in Table I: the graph associated with high quality articles is 12 times denser than the one associated with poor quality articles. Because comparing the density of very different size graphs might be legitimaly discussed, the average and the median degree are also reported: nodes associated with users collaborating in high quality articles have twice as many neighboors than those associated with poor articles. Based on these observations, we can suspect that high quality articles 2 As stated in Section IV-A the Wikipedia Editorial Team Assessment has manually labelled 30K articles. This enables the possibility to build some statistics on these categories. involve much more collaboration than poor quality articles. Thus, considering the collaborative process between users looks to be a promising track to assess the quality of an article.

Are the co-editors used to work together on top quality articles? Besides the density of the co-edit graphs, we would like to analyze the nature of the relations between the editors in order to determine in what extent authors collaborate together. Our intuition is that the higher the quality of an article is, the more frequent the interactions between co-editors are. The percentage of edges per weight per category is shown in Fig. 1. For readability purpose and because we aim at emphasizing on the impact of high weights, weights lower than 20 where omitted. Our intuition is obviously confirmed since high weights significantly appear more in the graph associated with high quality articles. Such an observation may indicate that good articles are authored by editors who should rely upon themselves since they are used to work together. Thus, considering the strength of the relation between co-editors looks to be a promising track to assess the quality of an article.

Contributions. We base our approach on the Mutual Reinforcement principle [START_REF] Hu | Measuring article quality in wikipedia: Models and evaluation[END_REF][8] [START_REF] Li | Automatically assessing wikipedia article quality by exploiting article-editor networks[END_REF] which assumes the interconnection between the author authority and the article quality and propose the following contributions:

• We propose a generic formulation of the Mutual Reinforcement principle. In this way, state-of-the-art approaches can be seen as instances of this general model. This makes the comparison much easier at a theoretical point of view;

• We instantiate our model by incorporating features extracted from the co-edit graph;

• We evaluated our approach and empirically demonstrate that it significantly perform better than the stateof-the-art approaches.

Paper Organization. Related work is discussed in section II.

Our proposed model is presented in section III. Section IV presents our experimental results. Finally, we conclude and give some perspectives in section V.

II. RELATED WORK

One of the earliest work [START_REF] Blumenstock | Size matters: Word count as a measure of quality on wikipedia[END_REF] has shown that the number of words in a document is a good predictor for article quality. More particulary, a specific class of articles in Wikipedia, known as Features articles, is easily predictable using the number of words in the article. Others correlations between simple features such as the number of edits and unique editors are empirically demonstrated [START_REF] Wilkinson | Cooperation and quality in wikipedia[END_REF]. While these results are persuasive, both previous works consider the task as a binary classification problem where all non Features articles are considered as negative examples. Various works make use of the notion of lifespan to infer the quality or thruthness of elements in the documents. Adler et al. [START_REF] Adler | A content-driven reputation system for the wikipedia[END_REF] introduce an approach that consists in measuring the reputation of the editors. The more an edit is preserved by subsequent reviewers, the more its author gains in reputation, and conversely, authors can lose reputation when their edits are reverted or deleted. The intuitive heuristic is that high-quality contributions survive longer throw the edit process because all subsquents editors implicitly approved the contribution simply by leaving it [START_REF] Biancani | Measuring the quality of edits to wikipedia[END_REF]. Hu et al. [START_REF] Hu | Measuring article quality in wikipedia: Models and evaluation[END_REF] also model the authority of the authors and the reviewers to compute the quality of each word in the article. Three models based on the mutual dependency between articles quality and contributors authority are proposed such as the Peer Review model which takes advantage of the implicit approbation of the reviewers, and the Prob Review model, which applies a decaying function over the authority of the reviewers around each approved word. Experiments show good results but are conducted over solely 200 documents. Moreover the structure between editors is not explicitly used. Adler et al. [START_REF] Adler | Assigning trust to wikipedia content[END_REF] also propose to credit each word with the reputation of the reviewers in proximity of the word and observe that words with low-trust assignment have high probability of being edited. In [START_REF] Javanmardi | Statistical measure of quality in wikipedia[END_REF], the quality of an article is modeled as a time-dependent function, allowing quality of articles to evolve during time. In their work, only two states are considered making the study specific to the binary classification problem. Wohner et al. [START_REF] Wöhner | Assessing the quality of wikipedia articles with lifecycle based metrics[END_REF] propose to use the editing intensity during a period of time but once gain, experiments are conducted over 200 articles and consider only two classes : Features articles i.e. good articles and articles for deletion i.e low quality articles. A more recent work [START_REF] Suzuki | Assessing quality score of wikipedia article using mutual evaluation of editors and texts[END_REF] reuses the mutual dependency concept between editors and texts and integrates the concept of lifespan as well as an adjustment of the authority of the reviewers in order to reduce the impact on text quality by vandal edits. A very recent work [START_REF] Li | Automatically assessing wikipedia article quality by exploiting article-editor networks[END_REF] explicitely formulates the mutual dependency between authors and articles with an Article-Editor network. The proposed model computes the quality of a document according to the editing relationships between article nodes and editor nodes using a variant of the PageRank algorithm [START_REF] Li | Automatically assessing wikipedia article quality by exploiting article-editor networks[END_REF]. Experiments are performed over only two classes of articles and none consider the relations between users. Finally, Suzuki [START_REF] Suzuki | Quality assessment of wikipedia articles using ¡i¿h¡/i¿index[END_REF] implicitly uses the structure of the co-edit network via the h-index measure in order to calculate the authority of the editors, but the quality of an article is directly computed using solely the derived authorities. In the conclusion, the author emits some doubts about the uniqueness of a quality function to distinguish good from bad articles. These approaches are summed up in Table II according to the type of relations they consider to address the article quality evaluation problem. Most of the works consider the task of assessing the quality of Wikipedia article as a binary classification problem whereas six different grades have been proposed by the Editorial Team of Wikipedia. Our work can interestingly deal with a non-predetermined number of classes by formulating the problem as a ranking problem. Moreover, none of the previous work has explicitly exploited the author interactions. However, as pointed out in Section I, some evidences exist to integrate the author interactions in the process. In this work we properly make use of these interactions in our model as described in the next section.

III. PROPOSED MODELS

Our work is grounded on 2 intuitions :

(1) Good articles are likely to be written by good editors, and conversely, good editors are more authorative if they participate, by writing and reviewing, good articles [7][12]. In our model, we capture this intuition postulating that the more authoritative users participate to the ellaboration of the article, the more the article is likely to be of good quality. The intrinsic dependency between quality and authority inevitably leads to an interdepedant pair of equations, where the quality of an article is defined over the qualities of its individuals piece of contents, and the authority of a user over the individual piece of contents he/she authored and approved. With our formulation, the amount of contribution of each editor in each article is catched in order to quantify the notion of implicit approvement, saying that if many authoritatives users who widely participate in an article leave previous edit on place it is because they judge it as good quality.

(2) Good articles are the result of a grouping of expert users. This second intuition seems to be particulary true for specialized topics. The proposed model quantifies in what extent these experts are used to collaborate together. Reviews between these authors are very expressive because probably much more trustworthy. Hence, we postulate that an article has a much more quality efficiency when these experts work together, and express that intuition using the co-edit graph between the authors. More formally, one can express this intuition using the probability P X (k) that two editors have together edited at least k articles belonging to a given class X of articles. To illustrate the intuition, let suppose 3 classes of articles A, B and C such as articles of class A are of better quality than those of type B and those belonging to class B of better quality than articles in class C. If editors of class A collaborate more than editors of class B and C, we should have

k P A (k) ≥ k P B (k) ≥ k P C (k).
It should be noted that this unequality might not hold for close classes on real data for all k. However, as illustrated in Fig. 1, it does hold when considering the two classes "good" and "poor" quality articles. The cumulative sum of edges weigth of good quality articles is indisputably greater than those of poor quality articles. This will be sufficient to push upward the scores of good quality articles.

A. Notations

Let X = {1, 2, ..., N } be the set of N articles and U = {1, 2, ..., M } the set of M users. Each article i is modeled by a sequence of char sequences x k i 1≤k≤ni , where n i is the number of sequences in the article. We denote by y i ∈ Y the quality of an article i. The author j of a sequence x k i is a k i , i.e., j = a k i . The set of sequences that a user j has authored is S(j).

Example III.1. Let X toy = {1, 2, 3, 4} be a set of 4 articles authored by U toy = {1, 2, 3, 4} a set of 4 editors. Corresponding char sequences are illustrated on Fig. 2. For instance, the article 1 is composed by 4 sequences x 1 1 , x 2 1 , x 3 1 , x 4 1 . The first one x 1 1 , i.e., the beginning of the article, has been authored by the user 1, i.e., a 1 1 = 1. The set of sequences that have been authored by the user 1 (in gray in the picture) is

S(1) = {x 1 1 , x 3 1 , x 2 2 , x 6 2 , x 2 3 }. a 1 1 = 1 a 2 1 = 2 a 3 1 = 1 a 4 1 = 3 a 1 2 = 2 a 2 2 = 1 a 3 2 = 3 a 4 2 = 2 a 5 2 = 2 a 6 2 = 1 a 3 1 = 2 a 2 3 = 1 a 3 3 = 2 a 4 3 = 3 a 1 4 = 4 a 2 4 = 3
Fig. 2. Four articles and their respective char sequences. For instance, article 4 (bottom) is composed of 2 sequences authored by users 4 and 3 respectively.

Let G = (V, E) be the co-edit graph defined by the set of nodes V = U and the edges set E, where edges e ij ∈ R + between each pair of users (i, j) ∈ U 2 measure the number of articles both users i and j have co-edited, formally :

e ij = |{k ∈ X : ∃ (u, v) : a u k = i ∧ a v k = j}| (1)
Example III.2. Let G toy = (U toy , E toy ) be the co-edit graph constructed using the previous example (see Fig. 3). The weight of the edge between 1 and 2 is e 1,2 = 3 because they have been co-authors in the articles 1, 2 and 3. Finally, the quality of a sequence x k i is Q k i and the authority of a user j is A j . Score calculations are discussed in the next sections.

B. Definitions

We first introduce the notion of lifespan of a sequence with is necessary to define the anterior and posterior neighborhood of a sequence and the notion of approvement.

Definition III.1. (Lifespan of a sequence) The lifespan l k i of a sequence x k i is the number of revisions it survives until the latest version of the article.

Definition III.2. (Anterior neighborhood)

The anterior neighborhood of a given sequence

x k i , denoted by N -(x k i ), is the set of sequences in the article i such that ∀x k ′ i ∈ N -(x k i ), l k ′ i > l k i . Definition III.3. (Posterior neighborhood)
The posterior neighborhood of a given sequence x k i , denoted by

N + (x k i ), is the set of sequences in the article i such that ∀x k ′ i ∈ N + (x k i ), l k ′ i < l k i . Definition III.4. (Approved sequence) The sequence x k
i is an approved sequence w.r.t. the user j (the user j has approved the sequence x k i ) if the user j has authored at least one sequence in the posterior neighborhood of 1 has been authored by the user 2 in the second revision), its lifespan is 3, i.e., l 2 1 = 3. The anterior neighborhood of the sequence

x k i , i.e, if ∃ x k ′ i ∈ S(j) ∩ N + (x k i ). Example III.
x 2 1 is N -(x 2 1 ) = {x 1 1 , x 3 1 } and its posterior neighborhood is N + (x 2 1 ) = {x 4 1 }. Finally, the sequence x 2 1
has been approved by the author 1.

a 1 1 = 1 a 1 1 = 1 a 2 1 = 2 a 3 1 = 1 a 1 1 = 1 a 2 1 = 2 a 3 1 = 1 a 4 1 = 3 a 1 1 = 1 a 2 1 = 2 a 3 1 = 1 a 1 1 = 1 a 2 1 = 2 a 3 1 = 1 a 4 1 = 1 First revision Latest revision Replace Insert Delete N -(x 2 1 ) N + (x 2

C. Model

When an authoritative user writes a content which is successively reviewed and approved by authoritative users, it is likely to be of good quality. This is even reinforced when many reviewers approved it and even more when these reviewers have widely participated to the article, e.g., the user 1 in Fig. 4. The more a reviewer throws himself/herself in an article, the more he/she is susceptible to see new edits and perform modifications if he/she judges the quality insufficient. This assumption becomes stronger when the number of authoritative reviewers increase. In other words, the amount of contributions of each reviewer seems to be fundamental to measure the concept of approvement. To formalize this intuition, the final quality Q k i of an individual sequence x k i is expressed by an approvement function which generically reflects the weighting schemes (or relations) between authors and reviewers. The quality of the sequence x k i is formally defined as:

Q k i = x k ′ i ∈ N + (x k i ) K a k ′ i →a k i A a k i , A a k ′ i ( 2
)
where K j→i : R + × R + → R + is the generic approvement function quantifying the implicit approvement by the user j of the sequence authored by the user i. Two forms of functions will be discussed in Section III-D. In a symmetric way, the authority A j of the author j is based on the quality of the sequences he/she authored and on those he/she approved and is defined as:

A j = x k i ∈ S(j) x k ′ i ∈ N -(x k i ) K j→a k ′ i Q k i , Q k ′ i (3)
Again, K must be instantiated in order to capture the approvement of a sequence by its reviewer. Finally, the global quality Q i of article i is simply defined over the quality of the sequences it contains in the latest revision, formally :

Q i = k≤ni Q k i ( 4
)
The computation of the model is described in Section III-E. It calculates for each article i a score Q i ∈ R + . The obtained list should be ranked to obtain the documents in decreasing (predicted) level of quality.

D. Approvement functions

The core of our model is the definition of the approvement act of a sequence by a reviewer. In this section, two approvement functions between an author i and a reviewer j are introduced and discussed:

K j→i 1 (a, b) = a + b (5)
K j→i 2 (a, b) = (ab) λ(1-eij ) (6)
Equation ( 5) (in short K 1 ) is a slight but crucial variation of the Peer Review model. In the original formulation [START_REF] Adler | A content-driven reputation system for the wikipedia[END_REF] the quantity Q k i is computed as the sum over the authority of the author and the authorities of each distinct reviewer who approved x k i . More formally, if {z 1 , z 2 , ..., z p } is the set of p distinct users (including the author herself/himself) who approved the sequence x k i , then the computed quality of the sequence is indeed :

Q k i = A z1 + A z2 + ... + A zp (7)
One can demonstrate that this expression is a result of a particular instantiation of our model.

Lemma 1. Let z 1 be the author of the sequence x k i and {z 2 , ..., z p } be the set of p -1 distinct reviewers who have approved x k i (including eventually the author himself/herself). Under our model, the use of the approvement function K 1 leads to a linear combination of the authority of the reviewers:

Q k i = θ 1 A z1 + θ 2 A z2 + ... + θ p A zp (8)
where ∀j > 1,

θ j = |{x k ′ i ∈ N + (x k i ) ∩ S(j)}| i.e
. number of sequences of user j bellonging to the posterior neighborhood N + (x k i ), and

θ 1 = |S + (x k i )| + |{x k ′ i ∈ N + (x k i ) ∩ S(1)}|.
Proof: Let N + (x k i ) = {x k1 i , x k2 i , ..., x ks i } be the set of s sequences authored by a set {z 1 , z 2 , ..., z p } of p distinct reviewers who approved sequence x k i . With K 1 to compute the quality Q k i , by definition :

Q k i = x k ′ i ∈ N + (x k i ) K a k ′ i →a k i (A a k i , A a k ′ i ) (9) = x k ′ i ∈ N + (x k i ) A a k i + A a k ′ i (10) = s • A a k i + A a k 1 i + A a k 2 i + A a k 3 i + ... + A a ks i (11)
By regrouping the identical reviewers, i.e., by identical a kj i

Q k i = s • A a k i + A a k 1 i θ1Az 1 + A a k 2 i + A a k 3 i θ2Az 2 +... + A a ks i θpAz p (12)
we finally obtained :

Q k i = θ 1 A z1 + θ 2 A z2 + ... + θ p A zp (13)
Hence, the Peer Review model is a particular case of our model assuming all θ i equals 1 (no weighting scheme is considered between the reviewers).

Our model enables the quality of a sequence to increase with the number of authoritative reviewers who have approved the sequence and also with the amount of contributions θ they have authored in the article. The desired intuition is captured: if many authoritative users who have predominantly participated to the article have approved a given sequence, its quality will fairly increase.

The co-edits relations between users are captured by equation (6) (K 2 in short). When e ij is close to 0 (none or very few relations between author j and reviewer i), the quality of a sequence increases only when both authors and reviewers are authoritative. Hence, unlike function K 1 , K 2 causes the approvement of an unauthoritative user to be almost unconsidered (final quantity is bounded by max(u i , u j ) because of the normalization of the authority score). Strong relations notably enable new registered users to rapidly gain in authority, and even more if they collaborate with authoritative users. To control the strength of the co-edit weights and consequently the quality scores, the user-parameter λ ∈ [0, 1] is introduced. When λ is close to 1, the function is elitist and tends to disfavor isolated users. Conversely, when λ is close to 0, the function is permissive and tends to encourage unauthoritative users. It should be noted that λ, being a hyperparameter, is not aimed to be learned but to be fixed before the execution of the algorithm in order to build a permissive or elitist function.

E. Calculation

The system formulated by the interdependent pair of equations 2 and 3 is solved by an iterative process that consists in alternatively computing authorities A and qualities Q. More details about the theoretical computation of the associated eigen values problem can be found in [START_REF] Golub | Matrix computations[END_REF]. The following generic process is used: 1) Initialize randomly both authorities and qualities scores 2) Compute quality scores with equation 23) Compute authority scores with equation 3 4) Normalize scores

The last three steps are repeated until convergence. Let Q t ∈ R N , resp. A t ∈ R M , be the vector of quality scores, resp. authority scores, at the t th iteration of the algorithm. The convergence is reached when d(Q t , Q t-1 ) + d(A t , A t-1 ) < ǫ, where d is a distance function and ǫ is aimed to control the convergence. In the experimentations, the L 2 norm was used as distance measure. For ǫ = 10 -3 , the convergence is rapidly reached (less than 10 iterations). Details about the computation are given by the following algorithms.

Algorithm 1 Quality(X )

1: for all i ∈ X do 2:

Q i ← 0 3:
for all x k i do 4:

Q k i ← x k ′ i ∈ N + (x k i ) K a k ′ i →a k i (A a k i , A a k ′ i ) 5: Q i ← Q i + Q k i 6:
end for 7: end for Algorithm 2 Authority(U )

1: for all u j ∈ U do 2:

A j ← 0 3:
for all x k i ∈ S(j) do 4:

A j ← x k ′ i ∈ N -(x k i ) K j→a k ′ i (Q k i , Q k ′ i ) 5:
end for 6: end for With a basic implementation of the anterior and posterior neighborhood search, the quality computation is performed in O(N n 2 ), where n = max i≤N {n i } and the Authority computation is performed in O(M sn) where s = max j≤M {|S(j)|}.

IV. EXPERIMENTS

This section is dedicated to the presentation of our result. We first properly introduced the methodology used for these experimentations. Quantitative results are then presented followed by a qualitative interpretation of two representative coedit graphs.

A. Protocol

Dataset description. We used a set of English documents from Wikipedia articles that have been reviewed by the Editorial Team Assessment of the WikiProject. Each article has been labelled according to the WikiProject quality grading scheme, and belongs to one of the following class Y = {S, C, B, GA, A, F A}. The user preferences are defined over Y as follows :

F A ≻ A ≻ GA ≻ B ≻ C ≻ S
The label S stands for Stub Articles (very bad quality articles with no meaningful content) while F A stands for Featured Articles (complete and professional articles). This scale is used as a ground truth for evaluation. It should be specified that labels were given according to the latest version of an article. We developed a crawler in Java to parse grades, topics and articles (from its first revision to the latest version). The history of edits are stored in a relational database. The raw dataset represents almost 130 Gb of text data. Statistics over these data are summarized in Table III. Preprocessing. A diff tool was developed in Python to extract the sets of sequences that survive until the last revision. During this preprocessing step, the lifespan of each sequence is updated. For each pair of consecutive revisions, the sequences are propagated, split and/or removed according to the possible sequence operations an editor can perform (replace, insert and delete characters). This preprocessing step is applied over a stratified random collection of the raw data of nearly 23,000 articles (a fix number of articles per category is randomly selected). More than 110,000 distinct users have produced around 2.8 million sequences. The co-edit graph built over this dataset is composed by more than 111,000 nodes and 5 millions of edges. Statistics about our dataset and resulted coedit graphs are synthesized in Table IV. Evaluation Metrics. Performances are evaluated using both standard ranking and classification evaluation metrics. To evaluate the ranking, the Normalized Discount Cumulative Gain at k (NDCG@k) was used [START_REF] Yining | A theoretical analysis of ndcg ranking measures[END_REF]. It computes a normalized score based on the degree of relevance of each document and a decaying function of their rank. In our case, the degree of relevance of a document is directly associated with its label : from 0 for documents belonging to class S (poor quality articles), to 5 for documents in class F A (very good articles).

A score equals to 1 indicating a perfect ranking. Once the permutation is computed by the model and documents ranked in decreasing order of (predicted) quality, one can split the list of documents in 6 categories according to the repartition of articles per grade in the ground truth. Hence, the number of positive examples per grade is evaluated using the Recall metric. A recall of 1 indicating a perfect classification.

Competitors. We compare our model to the following competitors [START_REF] Adler | A content-driven reputation system for the wikipedia[END_REF] :

Naive model. Final ranking is obtained by sorting articles by length.

Basic model. The effect of the reviewers is not taken in consideration.

Peer model. Reviewer effects are considered as important as the author ones. Final authority of an author is the sum of the authority of the distinct reviewers. Prob model. Authority of the reviewer is slightly decreasing with the distance between author and reviewer words. In the following experiments, the best decaying function f (d) =

1 max(0,d-β)+1 in [START_REF] Adler | A content-driven reputation system for the wikipedia[END_REF] was used. In the formulation, d is the distance between the words of the author and the closest word of the reviewer and β is a user-parameter to control the maximum distance over which authorities of reviewers are not fully considered. In the experiments, parameter β was set to 1000 (distance in characters), corresponding to the best run among different values of the parameter.

B. Quantitative experiments

The four competitors are compared with the two instantiations of our model, i.e., with approvement functions K 1 and K 2 . First, NDCG@k metric is used to compare the rankings over the articles at different levels, i.e., values of k are directly derived from the repartition of the article quality in the dataset (cumulative sum beyond the number of articles per class). Second, the recall for each of the 6 classes is used to compare to the competitors. The first class (FA) is evaluated using the first 245 documents, the second class (A) is evaluated using the next 51, and so on. Because K 2 depends on the parameter λ, only the best run (at least for k < 642) is used for the comparison (λ = 0, 7).

Experiments are conclusive: both proposed functions outperformed competitors for k ≤ 642. Both the amount of the contributions and the co-edits weights of the reviewers seem to be interested indicators to discriminate good to very good articles. Interestingly, the performances of K 1 using the NDCG@k (see Fig. 5) increase faster with k than K 2 , making the former globally more persuasive for k ≥ 642. The discrimination of articles of mid-quality using the co-edit relations seems to be more challenging. Nonetheless, when we examine the metrics considering all the articles (k = 22.423), K 2 again seems to benefit from the co-edit weights. The very few numbers of co-edit relations betweens authors of poor quality articles might be one factor which explains this sudden increase in the NDCG value.

This behavior can also be noted on Fig. 6. We plotted the evolution of the NDCG@k for the K 2 model in function of λ. We choosed to display the evolution for the top of the list documents (k = 245) and for the queue of the list (k = 3.600). The evolution clearly indicates that K 2 is much more competitive to discriminate very good articles for high values of λ (best NDCK@245 for λ = 0, 7), while the performances for the queue of the list (k = 3.600) remains globally constant. optimal as we just already said it, the separation induced by K 2 is incomparable with the others competitors.

It is interesting to see that the articles belonging to class A are very badly discriminated by every competitors (see Table V). Even if they are under-represented in the dataset (there are very few A articles in the English version of Wikipedia), they are even though considered in a drastically different way by K 1 and K 2 . By analyzing the ranks of each A article in the returned list by both models, we found that mean rank produced by K 1 is 1.194 (it roughly corresponds to B articles) while it is 382 for K 2 (closer to GA articles). Moreover, the dispersion of the ranks of A articles is 1.000 times smaller for the latter than for the former: considering co-editions seems to improve the precision.

Results confirm that combining different structural properties of the Wikipedia co-edit graph is beneficial and make the solution closer to the optimal solution : the co-edit graph is clearly discriminating to capture authoritative users and thus, good articles.

C. Qualitative interpretation

We now present the two co-edit graphs associated with a representative poor quality article (see Fig. 7 (left)) and Fig. 7. The filtered co-edit graphs of the articles related to A. Hillgruber and Kaga a top quality article (see Fig. 7 (right)) according to our proposed metric. The former is dedicated to A. Hillgruber3 , a conservative German historian, and has been labelled as a C-class article by the Editorial Team Assessment because of its partisanship. The latter is dedicated to Kaga4 , a Japanese aircraft career, and has been labelled as a FA-class article. Both graphs have been obtained using the following methodology: vertices are the union of the authors and their respective coeditors (in all the dataset). For ease of reading and because very weak co-edit relations are not of interest in this study, edges with weight equals 1 have been filtered out as well as subsequent isolated vertices. Table VI sums up some statistics on these two graphs. In the following we will refer to the nonfiltered graph related to A. Hillgruber, resp. Kaga, as G C , resp. G F A and to its filtered version as G ′ C , resp. G ′ F A . Since less than 1% of the edges have weights greater than 1, authors most often collaborate one time only whatever the quality of the article. This observation can though be sharpened by carefully analyzing the two graphs. On the one hand G ′

C

shows the following characteristics: (1) it is very sparse, (2) very few edges connect two author nodes, and (3) very fews non-author nodes remain in G ′ C after the filtering step. On the other hand G ′ F A shows some opposite characteristics: [START_REF] Cox | Truth in crowdsourcing[END_REF] it is denser than G ′ C , (2) much more edges connect two author nodes, and (3) more non-author nodes remain in G ′ F A after the filtering step. These observations confirm the soundness of our approach. Indeed, in poor quality article, the collaboration is punctual only; the added value of reviews is thus lower than for top quality articles where authors are more used to collaborate. Additionally, we compute the class of articles in which authors mostly participate. The repartition per class is given in the last line of Table VI. This shows that top quality articles, resp. low quality articles, are mostly written by authors who are used to write such good quality articles, resp. low quality articles. This is a clear evidence of the pertinence of the Mutual Reinforcement Principle.

V. CONCLUSION

Crowdsourcing platforms provide the possibility for anyone to freely contribute to their publicly available content. One inherent drawback of this collaborative process is the emergence of poor quality content. In this paper, we tackled the problem of automatically assessing articles quality in the particular case of Wikipedia. We proposed a generic formulation of the quality assessment problem based on the Mutual Reinforcement principle. We generalized previous works by introducing the notion of approvement functions which can take advantage of the relations between the editors. Such a formulation facilitates the theoretical comparison with stateof-the art approaches which can naturally be expressed as instances of our model. Motivated by some strong hints that legitimate the importance of considering the co-edit graph, two novel approvement functions were designed. The first function reinforces the quality of a content as a function of both the authority of the reviewers and the amount of their contributions in the article. The second function aims at capturing the relation between the authors and the reviewers since we have considered that the reviews of editors who are used to work together are more trustworthy. For this purpose, the co-edit network between editors was constructed and has appeared to have very interesting features. Experiments conducted on real Wikipedia articles are very conclusive. The proposed model, by improving state-of-the-art methods, empirically confirmed our two intuitions and open several perspectives.

In future work, we first plan to extend our model by generalizing the notion of neighborhood. Indeed, we think it would be beneficial to consider both horizontal (time) and vertical (documents) aspects of the neighborhood of a sequence. Notably, such an operator would enable to even more generalize our model and to reformulate other state-of-the-art works, e.g., the Prob Review model. Scalability is a major concern. We plan to study the possibility to adapt our model to a Big Data environment using some parallelization strategies.

Finally, because of the intensive edit rate of Wikipedia articles, adapting our model to a streaming environment would enable the quality calculation on the fly.

Fig. 1 .

 1 Fig. 1. The histogram of edges per weight on high quality articles (white) and poor quality articles (black).

Fig. 3 .

 3 Fig. 3. Representation of the co-edit graph Gtoy associated with Xtoy and Utoy. For instance, e 4,3 is 1 because users 4 and 3 have co-edited only one article (article 4).

3 .

 3 The above defined concepts are illustrated on Fig. 4. It illustrates the edit process of the article 1 from the initial commit of the user 1 (top of the figure) to the latest revision (bottom of the figure). Let us consider the sequence x 2 1 in the latest version of the article. Since it appears three revisions before the latest version of the article (the sequence x 2

1 )Fig. 4 .

 14 Fig. 4. Illustration of an edit history of an article. Neighborhoods of sequence x 2 1 are computed considering the latest version of the article.

Fig. 5 .

 5 Fig. 5. Evaluation with the NCDG@k metric for k ∈ {245, 296, 642, 1.654, 3.600, 22.423} of the competitors and the proposed model with 2 approvement functions K 1 and K 2 .

Fig. 6 .

 6 Fig. 6. Evaluation of the proposed model using approvement function K 2 for different values of λ.

TABLE I

 I

	.	SOME STATISTICS ABOUT THE CO-EDIT GRAPHS PER
		ARTICLE QUALITY LEVEL
		Type of articles	Poor quality	High quality
		Articles	18,823	245
		Editors	36,973	9,110
		Collaborations	369,844	546,273
		Density (10 -5 )	0.53	6.8
		Avg. Degree	15	26
		Med. Degree	5	10

TABLE III

 III

			.	RAW DATASET STATISTICS
	Grade	y i	Articles	Revisions	Authors	Gb
	FA	5	611		765,917	174,178	29
	A	4	67		32,700	7,492	2
	GA	3	462		398,757	112,266	15
	B	2	1.459	1,283,225	330,010	44
	C	1	3.382	1,521,416	423,021	32
	S	0	26.736	1,164,376	236,715	7

TABLE IV

 IV

		.	DATASET
	Grade	FA	A	GA	B	C	S
	Articles	245	51	346	1,012	1,946	18,823
	Authors per article (mean)	61	37	37	46	41	7
	Lifespan per article (mean)	275	166	154	141	126	12
	Sequences (mean)	1,114	963	809	695	439	40
	Sequences length (mean)	78	44	86	85	88	73
	Nodes (10 3 )	9.11	1.41	8.54	28.3	48.1	36.9
	Edges (10 3 )	546	62	406	1,505	2,279	369

TABLE V

 V

			NDCG@k
	1
	0.8
	0.6
	0.4
	0.2
	0
	k=245	k=296	k=642	k=1.654	k=3.600	k=22.423
		Naive	Basic	Peer	Prob	K 1	K 2
							.	RECALL PER CLASS
					Model	FA	A	GA	B	C	S
					Naive	25.30	3.92	11.56	32.21	59.66	98.68
					Basic	0.4	0	0.86	13.93	14.33	90.99
					Peer	28.16	1.96	3.17	28.45	55.3	98.95
					Prob	20.81	1.96	4.33	27.76	39.77	96.99
					K1	61.22	3.92	10.98	34.68	53.85	98.07
					K2	75.91	3.92	67.91	90.21	95.94	99.82
				Finally, Table V summarizes the capacity of the models to
				discriminate the six class of articles. Even if the ranking is not

TABLE VI .

 VI SOME STATISTICS ABOUT THE CO-EDIT GRAPHS OF THE ARTICLES RELATED TO A. Hillgruber AND Kaga

		A. Hillgruber	Kaga
	#vertices	42572	14023
	#edges	128322	55368
	#authors	63	58
	#vertices (filtered)	70 (0.17%)	93 (0.66%)
	#edges (filtered)	87 (0.07%)	420 (0.76%)
	Class repartition	FA: 4.3 / A: 0 / GA: 5.6	FA: 55.7 / A: 0.8 / GA: 3.6
	(%)	B: 1.4 / C: 85 / S: 3.7	B: 25.3 / C: 8 / S: 6.6

http://en.wikipedia.org/wiki/Wikipedia:Statistics

https://en.wikipedia.org/wiki/Andreas Hillgruber

https://en.wikipedia.org/wiki/Japanese aircraft carrier Kaga


Célia Martinie
email: martinie@irit.fr

Philippe Palanque
email: palanque@irit.fr

Design, Development and Evaluation Challenges for Future Mobile User Interfaces in Safety-Critical Contexts

Keywords: H.5.2 User Interfaces Design, Reliability, Human Factors Mobile user interfaces, interactive critical systems, design and development processes

The large variety of commercialized mobile devices, their reduced costs and their increased level of maturity makes them suitable for exploitation in an increasing number of contexts. However, the idiosyncrasies of interactive systems on mobile platforms raise many new challenges in terms of design, development and use. Such specific aspects include, dynamicity of contexts of use, interaction on the move and device management (e.g. power availability) that bring together many challenges and opportunities in every stages of the development process. This includes, for instances, multiple devices issues such as designing a single application to be used on various mobile (or not) devices as well as designing various applications to be deployed on multiple devices to be used by a single user. Several research contributions started to tackle these challenges but few of them address the additional constraints raised by safety-critical systems. While in the domain of interactive systems usability and user experiences are the first class citizens, safety critical systems designs call for well designed and implemented trade-offs between multiple properties such as safety, reliability… in addition to usability and UX. This position paper aims at presenting new challenges for designing, developing, evaluating and deploying future mobile UIs targeting at the domain of critical systems.

INTRODUCTION

The large variety of commercialized mobile devices, their reduced costs and their increased level of maturity makes them suitable for exploitation in an increasing number of contexts. However, the idiosyncrasies of interactive systems on mobile platforms raise many new challenges in terms of design, development and use. Such specific aspects include, dynamicity of contexts of use, interaction on the move and device management (e.g. power availability) that bring together many challenges and opportunities in every stages of the development process. This includes, for instances, multiple devices issues such as designing a single application to be used on various mobile (or not) devices as well as designing various applications to be deployed on multiple devices to be used by a single user. Several research contributions started to tackle these challenges, from the design and/or development perspectives but few of them address the additional constraints raised by safety-critical systems. Several existing studies close to this topic are mostly dealing with wearable UIs for aircraft maintenance [START_REF] Avizienis | Basic concepts and taxonomy of dependable and secure computing[END_REF] [11] [START_REF] Witt | Designing a Wearable User Interface for Hands-free Interaction in Maintenance Applications[END_REF]. Their motivation are based on two main points:

-Aircraft maintenance activities are highly procedural and ruled, and they require the elicitation and application of relevant knowledge according to the diagnostic.

-Aircraft maintenance users (also called operators) have to perform actions and to manage information across several contexts.

These studies highlight the relevance and benefits of using crossdevices mobile UIs for performing monitoring and control activities on critical systems. Furthermore, recent integration of mobile devices in critical contexts (electronic flight bags in the cockpit replaced paper flight bags1 ) highlight the interest of safety-critical industries for new mobile technologies, but also highlights the fact that some of the users of critical systems will probably interact more and more with mobile applications to perform their job.

While in the domain of interactive systems usability and user experiences are the first class citizens, safety critical systems designs call for well designed and implemented trade-offs between multiple properties such as safety, reliability, security, dependability, … in addition to usability and UX. This position paper aims at presenting new challenges for designing, developing, evaluating and deploying future mobile UIs targeting at the domain of critical systems.

DESIGN CHALLENGES FOR MOBILE UIs IN CRITICAL CONTEXTS

Future mobile UIs will have to address multi-device interactions and collaborative work. Operators of critical systems will continue to interact with several devices and to collaborate with other partners of their organization. Conceptual frameworks have been proposed to start addressing these issues [16] [13]. We extracted from these framework design considerations that are relevant for discussion with regards to the particular case of interactive critical system design. Operators of interactive critical systems have particular procedures and tasks to follow. They have to reach a predetermined level of knowledge (declarative and procedural) before being granted qualification to operate with the systems.

Future mobile UIs for cross-platform applications will have to address appropriate composition [START_REF] Wäljas | Cross-platform service user experience: a field study and an initial framework[END_REF] of presented information and commands. In critical contexts, designers will have to identify which tasks are relevant to be migrated on which type of mobile device (wearable, handheld…) and which tasks are not relevant to be migrated on mobile devices.

Future mobile UIs for cross-platform applications will have to address task continuity across several mobile UIs [START_REF] Wäljas | Cross-platform service user experience: a field study and an initial framework[END_REF]. In critical contexts, this concept may not be relevant for all the tasks. Tasks are planned and related to a particular procedure and context. Designers will have to provide solutions to determine which tasks could be seamlessly performed across different mobile UIs and which tasks should be dedicated to be performed on particular mobile UI.

Future mobile UIs for cross-platform applications will have to address consistency between mobile UIs [START_REF] Wäljas | Cross-platform service user experience: a field study and an initial framework[END_REF].

In critical contexts, the designers will have to investigate whether or not it is appropriate to provide only fully consistent mobile UIs. Operators of critical mobile UIs will have to execute planned procedures and some of the procedures can be bound to dedicated contexts (routine activities, failure detections and recovery…) [START_REF] Martinie | A multi-formalism approach for model-based dynamic distribution of user interfaces of critical interactive systems[END_REF]. In order to ensure that there will not be ambiguities for the operator when performing a particular kind of procedure, the designer may have to provide distinct (non-consistent) UIs presenting different behaviors and compositions as a way to ensure that there will be no ambiguity for the operator concerning the current type of on-going procedure.

Future mobile UIs for cross-platform applications will have to address simultaneous collaborative interaction [START_REF] Sørensen | The 4C framework: principles of interaction in digital ecosystems[END_REF]. The designers will have to deal with the coordination and communication activities between operators using mobile UIs. In critical contexts, activities can be highly influenced by time constraints, organizational issues and planned workflows between inter and intra organizational entities [START_REF] Martinie | Multi-models-based engineering of collaborative systems: application to collision avoidance operations for spacecraft[END_REF].

Future mobile UIs for cross-platform applications will have to address communality of the mobile systems and of the information [START_REF] Sørensen | The 4C framework: principles of interaction in digital ecosystems[END_REF]. The designers of the mobile UIs will have to investigate how the usage of a range of mobile systems (glasses, watches, smartphones, tablets…) and associated UIs may fit with team activities. How will these artifacts be shared amongst the team and how the other members of the team will be provided with the awareness of which member is using which mobile system (and for executing which procedure)?

Future mobile UIs for cross-platform applications will have to address the extra activities related to the monitoring of mobile systems and UIs, which will come in addition to the planned operators tasks.

Operators will have to ensure that their mobile systems and UIs are in a functional state while accomplishing their mission. Designers will have to deal with this additional task type: monitoring the vital parameters of all the mobile systems they use to accomplish their tasks (with for example a centralized way to get information about power level, connection status… for all of the in use mobile systems and UIs).

DEVELOPMENT CHALLENGES FOR MOBILE UIs IN CRITICAL CONTEXTS

Future mobile UIs will have to go through critical systems development processes. They will have to fulfill dependability requirements such as reliability and security. Solutions have been proposed to discuss possible backup strategies for mobile devices [START_REF] Avizienis | Basic concepts and taxonomy of dependable and secure computing[END_REF]. In order to discuss the application of these strategies to mobile systems and UIs, we extracted development issues that are relevant for future mobile UIs.

Future mobile UIs and applications in critical contexts will have to fulfill reliability requirements. Several existing mechanisms from dependable computing field could be applied, such as redundancy and diversity.

Designers and developers will have to investigate whether these mechanisms may be applied or whether they have to be tuned. For example, an operator may get granted 2 handheld devices for the same purpose (including a backup one). The backup one would be connected to the regular one so that each executed command on the first device is also automatically executed on the second. Then, in case that the first handheld device gets broken or nonfunctional, the operator can use the second backup device and continue her/his task without loss of context and with a continuity of service. Intra and inter devices redundancy concepts have then to be studied in detail for future mobile UIs.

Future mobile UIs and application will have to fulfill security requirements. If operators of critical systems have their applications on the go, what will prevent malicious usage of their devices? For example, an oncall duty operator of a satellite mission, responsible for remotely monitoring the status of the satellite regularly during the week-end, could bring her/his mobile device to the mall and have the device stolen. What if the mobile UI enables to send telecommands to the satellite? Designers and developers have to investigate which commands and interactions should be available in the mobile UI according to the context of use.

Future mobile UIs and applications will also have to address potential conflicts and interferences between professionals and personal devices [START_REF] De Sá | Designing and Evaluating Mobile Interaction: Challenges and Trends[END_REF].

In addition, to dependability requirements that future mobile UIs and systems will have to fulfill, developers will have to face the diversity of mobile systems and software platforms. Several approaches have been proposed and partially answer to this issue. Model-driven approaches have be coined to provide support for developing UIs for multiple devices (mobile or not) [START_REF] Paternò | A logical framework for multidevice user interfaces[END_REF]. Software product lines have also been proposed to provide support for development of similar software products across devices [START_REF] Pleuss | User interface engineering for software product lines: the dilemma between automation and usability[END_REF]. And engineering techniques and framework have been proposed for developing distributed UIs [START_REF] Nebeling | Interactive development of cross-device user interfaces[END_REF]. But they are not sufficient to overcome new design challenges as they don't provide support to all of the here above mentioned designed and development challenges.

EVALUATION CHALLENGES FOR MOBILE UIs IN CRITICAL CONTEXTS

The software and hardware platforms on which the future mobile UIs will be deployed will have an impact on the performance of the UI and then on the performance of the users. Existing evaluation techniques will have to be enhanced to take into account the diversity of platforms across multiple interaction contexts [START_REF] De Sá | Designing and Evaluating Mobile Interaction: Challenges and Trends[END_REF]. In critical contexts, other properties (such as reliability of the system and of the operator) will have to be measured. The proposed evaluation methods will then have to also encompass techniques and tools which provide support for measuring them across multiple mobile UIs and contexts.

CONCLUSION AND PERSPECTIVES

This position paper presented design, development and evaluation challenges for future mobile UIs in critical contexts. Beyond the discussed issues, other key points will also have to be dealt with in order to propose solutions for deployment of mobile UIs in critical contexts. For example, will existing verification techniques (such as formal methods) be applicable to future crossplatform mobile UIs or will they have to be tuned? How the designers and developers will ensure that future mobile UIs are not more error prone than the existing UIs for critical systems? Will the existing training design and development methods be sufficient for these future mobile UIs?

AUTHORS' BACKGROUND AND INTERESTS

The authors have a long term experience in the area of interactive critical systems2 including design and implementation issues of command and control systems. We have been working closely with main actors in the area of safety critical interactive systems including air-traffic management, aircraft cockpits and satellite ground segments. New mobile systems and UIs are not yet widely available in these types of systems as the difficulties in terms of reliability, dependability and certification limiting their use to non-critical aspects of the operation of these systems. However, increased maturity of the hardware platforms of mobile systems might make them amenable to a wider exploitation in these contexts. Our industrial partners (in these safety critical contexts) start to assess how far these systems are from the required dependability levels and how much their use would improve operations. Participation to the workshop would be beneficial to us by confronting our expertise in safety critical systems with potential research directions for future UIs in mobile contexts. It

http://www.windowscentral.com/lufthansa-latest-airline-equippilots-surface-pro-3

http://www.irit.fr/recherches/ICS/index.html is important to note that one of the authors has spent 8 years in R&D activities at Motorola Mobile Devices (see patents in the reference list[5][15]).


𝑎,𝑏 M Lisowska
email: marta.lisowska@cern.ch

Y Angelis 𝑐

S Aune 𝑑

J Bortfeldt 𝑒

F Brunbauer 𝑎

𝑐 , K. Dehmelt E Chatzianagnostou

D 𝑓

𝑑 G Desforge

𝑔 K J Fanourakis

Floethner 𝑎

𝑖 M Gallinaro

Garcia 𝑗

P Garg

𝑑 I Giomataris

K Gnanvo 𝑘

𝑙 T Gustavsson

F J Iguaz 𝑑

𝑎,𝑚,𝑛 D Janssens

A Kallitsopoulou 𝑑

𝑜 M Kovacic

𝑑 P Legou

𝑝 J Liu

M Lupberger

Y Meng 𝑝

𝑞 H Muller 𝑎

E Oliveri 𝑎

𝑟 G Orlandini 𝑎

𝑑 T Papaevangelou

𝑠 M Pomorski

L Ropelewski 𝑎

𝑐,𝑡 , L. Scharenberg 𝑎,𝑞 , T. Schneider 𝑎 D Sampsonidis

E Scorsone 𝑠

𝑑 L Sohl

𝑎 M Van Stenis

Y Tsipolitis 𝑢

𝑡 S E Tzamarias 𝑐

A Utrobicic 𝑤

𝑥 R Veenhof 𝑎

𝑝 X Wang

𝑦 S White 𝑎

Z Zhang

Y Zhou

Towards robust PICOSEC Micromegas precise timing detectors

A

: The PICOSEC Micromegas (MM) detector is a precise timing gaseous detector consisting of a Cherenkov radiator combined with a photocathode and a MM amplifying structure. A 100channel non-resistive PICOSEC MM prototype with 10×10 cm 2 active area equipped with a Cesium Iodide (CsI) photocathode demonstrated a time resolution below 𝜎 = 18 ps. The objective of this work is to improve the PICOSEC MM detector robustness aspects, i.e. integration of resistive MM and carbon-based photocathodes, while maintaining good time resolution. The PICOSEC MM prototypes have been tested in laboratory conditions and successfully characterised with 150 GeV/c muon beams at the CERN SPS H4 beam line. The excellent timing performance below 𝜎 = 20 ps for an individual pad obtained with the 10×10 cm 2 area resistive PICOSEC MM of 20 MΩ/ showed no significant time resolution degradation as a result of adding a resistive layer. A single-pad prototype equipped with a 12 nm thick Boron Carbide (B 4 C) photocathode presented a time resolution below 𝜎 = 35 ps, opening up new possibilities for detectors with robust photocathodes. The results made the concept more suitable for the experiments in need of robust detectors with good time resolution.

K

: Micropattern gaseous detectors (MSGC, GEM, THGEM, RETHGEM, MHSP, MI-CROPIC, MICROMEGAS, InGrid, etc), Cherenkov detectors, Timing detectors The development of technologies for precise timing detectors has been driven by the demanding environments expected in future High Energy Physics experiments. The requirements of a time resolution of tens of picoseconds, stable long-term operation and a large area coverage must be met to make the device suitable. Within the PICOSEC Micromegas (MM) collaboration, a gaseous detector designed to achieve precise timing response is being developed [START_REF] Bortfeldt | for the PICOSEC Micromegas Collaboration, PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector[END_REF][START_REF] Aune | for the PICOSEC Micromegas Collaboration, Timing performance of a multi-pad PICOSEC-Micromegas detector prototype[END_REF][START_REF] Lisowska | for the PICOSEC Micromegas Collaboration, Sub-25 ps timing measurements with 10×10 cm 2 PICOSEC Micromegas detectors[END_REF]. First proof-of-concept singlepad prototypes demonstrated a time resolution below 𝜎 = 25 ps [START_REF] Bortfeldt | for the PICOSEC Micromegas Collaboration, PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector[END_REF]. A recent multipad PICOSEC MM indicated how precise time resolution can be achieved with a non-resistive 100-channel gaseous detector with 10×10 cm 2 active area equipped with a CsI photocathode, showing a time resolution below 𝜎 = 18 ps [START_REF] Lisowska | for the PICOSEC Micromegas Collaboration, Sub-25 ps timing measurements with 10×10 cm 2 PICOSEC Micromegas detectors[END_REF]. Although the first tests of alternative, more robust approaches have been performed in the past (i.e. timing measurements of single-pad prototypes with resistive MM and Diamond-Like-Carbon (DLC) photocathodes [START_REF] Sohl | Development of PICOSEC-Micromegas for fast timing in high rate environments[END_REF]), new developments including the integration of resistive MM in 100-channel modules and robust Boron Carbide (B 4 C) photocathodes have not been studied up to now. The objective of this work is to improve the PICOSEC MM detectors robustness aspects while maintaining a good time resolution.

Detection concept

The PICOSEC MM detection concept [START_REF] Bortfeldt | for the PICOSEC Micromegas Collaboration, PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector[END_REF] is illustrated in Fig. 1 (a). A charged particle passing through a Cherenkov radiator, creates a cone of ultraviolet (UV) photons which are converted into primary electrons on a photocathode coated directly on the radiator. Due to a high electric filed, the extracted electrons successively ionise the gas molecules, causing the multiplication of the electrons, first in the preamplification gap, and then, after passing through the MM mesh, in the amplification gap. A gas mixture of 80% Neon, 10% CF 4 and 10% Ethane at ambient pressure is used to fill both regions. The amplified electrons induce a signal on the anode which is passed through an amplifier and read out by a digitizer. A typical PICOSEC MM waveform consists of a fast electron peak and a slow ion tail, as presented in Fig. 1 (b). The rising edge of the electron peak is used to extract the time of arrival of the charged particle.

Experimental setup

The PICOSEC MM prototypes have been tested for stable operation in laboratory conditions and successfully characterisied at the CERN SPS H4 beam line with 150 GeV/c muon beams during RD51 test beam campaigns. The main purpose of the muon beam characterisation was to measure the time resolution of the detectors. The experimental setup was based on a beam telescope with triggering, timing and tracking capabilities. Triple-GEMs were used to obtain the precise tracking of the particles, while a micro-channel plate photomultiplier tube (MCP-PMT, Hamamatsu R3809U50) was used as timing reference (time resolution in the inner part of the active area below 𝜎 = 4 ps [START_REF] Sohl | Spatial time resolution of MCP-PMTs as a t 0 -reference[END_REF]) and data acquisition trigger. As the new electronics dedicated for multipad device, custom-made RF pulse amplifier cards optimised for PICOSEC [START_REF] Utrobicic | for the PICOSEC Micromegas Collaboration, A large area 100 channel PICOSEC Micromegas detector with sub 20 ps time resolution[END_REF] and 128-channel SAMPIC Waveform Time to Digital Converter [START_REF] Breton | Measurements of timing resolution of ultra-fast silicon detectors with the SAMPIC waveform digitizer[END_REF] with 8.5 GS/s sampling frequency were used.

Resistive Micromegas

Resistive MM are essential to make the PICOSEC concept suitable for physics applications. The advantages of using resistive MM in the PICOSEC detector include the limitation of the destructive effect of discharges, resulting in stable operation under intense particle beams, as well as a possibly better position reconstruction and signal sharing. A possible risk of a resistive MM implementation may be to affect the shape of the signal, in particular the rising edge, by spreading the signal due to the resistive layer, which could result in a loss of the timing information. The objective is to profit from the advantages of the resistive MM while maintaining a good time resolution. To choose an optimal resistivity, two aspects must be considered. Firstly, the resistivity must be low enough to minimise the voltage drop during high-rate beam and improve the position reconstruction using the signal weighted average from neighbouring pads. Secondly, the resistivity needs to be high enough to ensure stable operation and not affect the rising edge of the signal. Simulations of rate capability and signal rising edge dependence were performed to select the resistivity for the new PICOSEC prototype. Informed by the simulations, a 100-channel detector with a 10×10 cm 2 area resistive MM with anode surface resistivity of 20 MΩ/ was produced. The production procedure was done in the same way as for a non-resistive multipad [START_REF] Lisowska | for the PICOSEC Micromegas Collaboration, Sub-25 ps timing measurements with 10×10 cm 2 PICOSEC Micromegas detectors[END_REF] with an additional step of adding a resistive layer of DLC that was placed on top of the readout pads with an insulating layer in between.

First test beam measurements of the multipad equipped with a resistive MM of 20 MΩ/ , a CsI photocathode and RF pulse amplifiers were performed with an oscilloscope to compare the results with a non-resistive prototype [START_REF] Lisowska | for the PICOSEC Micromegas Collaboration, Sub-25 ps timing measurements with 10×10 cm 2 PICOSEC Micromegas detectors[END_REF]. The time resolution of the detector was calculated as the standard deviation of the signal arrival time (SAT) distribution. The measurements of the 10×10 cm 2 resistive PICOSEC MM of 20 MΩ/ showed a time resolution below 𝜎 = 20 ps for an individual pad, as can be seen in Fig. 2. The results confirm that adding a resistive layer does not affect the time resolution of the detector to a significant extent. To characterise the full detector, the SAMPIC digitiser was used [START_REF] Breton | Measurements of timing resolution of ultra-fast silicon detectors with the SAMPIC waveform digitizer[END_REF]. The objective was to measure the timing precision. The measurements of the time resolution across the 10×10 cm 2 area resistive PICOSEC MM of 20 MΩ/ are presented in Fig. 3. First results obtained with the new digitizer showed uniform timing response within the pads and a narrow distribution of the time resolution across the channels with the mean value of 𝜎 ≈ 23 ps, proving SAMPIC to be a suitable tool to study the response of 100-channel PICOSEC detector.

Robust photocathodes

The base UV-to-electron converter for the PICOSEC MM detector is a semi-transparent CsI photocathode. The high quantum efficiency of CsI leads to the production of around 10 photoelectrons per minimum ionizing particle (for a 3 mm thick MgF 2 radiator with a 3 nm Cr layer and a 18 nm CsI photocathode [START_REF] Bortfeldt | for the PICOSEC Micromegas Collaboration, PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector[END_REF]). However, CsI photocathodes can be easily damaged by ion back flow, sparks, discharges and are sensitive to humidity. Therefore, there is a need to search for alternative, more robust photocathode materials. The most promising candidates are B 4 C, DLC and nanodiamonds.

The measurements of the time resolution for different thicknesses of B 4 C photocathodes were performed during the RD51 test beam campaigns. A single-channel non-resistive PICOSEC MM prototype was used to test the samples. Different photocathode thicknesses ranging from 2 nm to 14 nm were tested. The best timing response was obtained for a 12 nm thick B 4 C sample. First measurements from a single-pad detector with 120 µm preamplification gap equipped with a 12 nm thick B 4 C photocathode, presented in Fig. 4

Conclusions

The work described in this paper focused on improving the robustness aspects of the PICOSEC MM detectors, including the integration of resistive MM and robust photocathodes, while maintaining good time resolution. The excellent timing performance below 𝜎 = 20 ps for an individual pad obtained with the 10×10 cm 2 area resistive PICOSEC MM of 20 MΩ/ equipped with a CsI photocathode showed no significant time resolution degradation as a result of adding a resistive layer. Measurements performed with a scalable readout chain including the SAMPIC digitiser resulted in a successful readout of a multi-channel detector. A single-pad prototype equipped with a 12 nm thick B 4 C photocathode presented a time resolution below 𝜎 = 35 ps, opening up new possibilities for detectors with robust photocathodes. The results made the concept more suitable for the experiments in need of robust detectors with precise timing. Developments towards applicable detectors are ongoing. The challenges include stable operation in intense beams with resistive MM Multipad and operating a PICOSEC MM detector with a 10×10 cm 2 area B 4 C photocathode. Scaling up the PICOSEC MM detector by tiling 10×10 cm 2 modules or the development of larger prototypes are potential next development steps.

Figure 1 .

 1 Figure 1. (a): PICOSEC MM detection concept. A charged particle passing through a Cherenkov radiator creates UV photons that are converted into electrons on a photocathode, then multiplied in the preamplification gap and afterwards in the amplification gap. (b): A typical PICOSEC MM waveform consisting of a fast electron peak and a slow ion tail.

Figure 2 .

 2 Figure 2. SAT distribution of one channel of the resistive PICOSEC MM. The cathode voltage was set at -490 V, the MM mesh at ground and the anode at +290 V. The histogram consists of the data after implementing a geometrical cut of a 4 mm diameter circle around the pad center to include only fully contained events. A Gaussian fit of the data gives a time resolution of the measured channel of 𝜎 = 19.2 ps.

Figure 3 .

 3 Figure 3. Time resolution for the 10×10 cm 2 area resistive PICOSEC MM measured using SAMPIC digitiser. Note that only 66 out of 100 pads were measured. (a): 3D plot of the timing response across the channels. (b): Histogram of the time resolution RMS for all measured channels yields the mean value 𝜎 ≈ 23 ps.

  photocathode[START_REF] Bortfeldt | for the PICOSEC Micromegas Collaboration, PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector[END_REF]). However, CsI photocathodes can be easily damaged by ion back flow, sparks, discharges and are sensitive to humidity. Therefore, there is a need to search for alternative, more robust photocathode materials. The most promising candidates are B 4 C, DLC and nanodiamonds.The measurements of the time resolution for different thicknesses of B 4 C photocathodes were performed during the RD51 test beam campaigns. A single-channel non-resistive PICOSEC MM prototype was used to test the samples. Different photocathode thicknesses ranging from 2 nm to 14 nm were tested. The best timing response was obtained for a 12 nm thick B 4 C sample. First measurements from a single-pad detector with 120 µm preamplification gap equipped with a 12 nm thick B 4 C photocathode, presented in Fig.4, showed a time resolution below 𝜎 = 25 ps. The measurements performed 20 hours later presented a time resolution below 𝜎 = 35 ps. The 12 nm thick B 4 C sample demonstrated the best time resolution achieved for robust photocathode materials studied up to now.

Figure 4 .

 4 Figure 4. SAT distribution of a single-pad prototype equipped with a 12 nm thick B 4 C photocathode. The voltage on the cathode was set at -490 V and on the anode at +275 V. A Gaussian fit yields a time resolution of 𝜎 = 24.3 ps at the beginning (a) and 𝜎 = 34.4 ps after 20 hours of operation with gas flushing (b), respectively.

Acknowledgments

We acknowledge the support of the CERN EP R&D Strategic Programme on Technologies for Future Experiments; the RD51 collaboration, in the framework of RD51 common projects; the Cross-Disciplinary Program on Instrumentation and Detection of CEA, the French Alternative Energies and Atomic Energy Commission; the PHENIICS Doctoral School Program of Université Paris-Saclay, France; the Program of National Natural Science Foundation of China (grant number 11935014); the COFUND-FP-CERN-2014 program (grant number 665779); the Fundação para a Ciência e a Tecnologia (FCT), Portugal (CERN/FIS-PAR/0005/2021); the Enhanced Eurotalents program (PCOFUND-GA-2013-600382); the US CMS program under DOE contract No. DE-AC02-07CH11359.


Peder Rodrigue Garcia
email: rodrigue.garcia@chu-poitiers.fr

Peder Emil Warming

Kumar Narayanan

Pascal Defaye

Laurence Guedon-Moreau

Hugues Blangy

Olivier Piot

Christophe Leclercq

Eloi Marijon

Laurence Guedon Moreau

Garcia Rodrigue

Laurence Guedon-Moreau6

Hugues Blangy7

Dynamic changes in nocturnal heart rate predict short-term cardiovascular events in patients using the wearable cardioverter-defibrillator: from the WEARIT-France cohort study

Keywords: Wearable cardioverter-defibrillator, Heart rate, Remote monitoring, Pre-emptive action, Heart failure NCT03319160

AIMS

While elevated resting heart rate measured at a single point of time has been associated with cardiovascular outcomes, utility of continuous monitoring of nocturnal heart rate (NHR) has never been evaluated. We hypothesized that dynamic NHR changes may predict, at short term, impending cardiovascular events in patients equipped with a wearable cardioverter defibrillator (WCD).

Methods and Results

The WEARIT-France prospective cohort study enrolled heart failure patients with WCD between 2014 and 2018. Nighttime was defined as midnight to 7 a.m. NHR initial trajectories were classified into four categories based on mean NHR in the first week (High/Low) and NHR evolution over the second week (Up/Down) of WCD use. The primary endpoint was a composite of cardiovascular death and heart failure hospitalization. A total of 1013 [61 (interquartile range, IQR 53-68) years, 16% women, left ventricular ejection fraction 26% (IQR 22-30)] were included. During a median WCD wear duration of 68 (IQR 44-90) days, 58 patients (6%) experienced 69 events. After considering potential confounders, High-Up NHR trajectory was significantly associated with the primary endpoint compared to Low Down [adjusted hazard ratio (HR) 6.08, 95% confidence interval (CI) 2.56-14.45, P < 0.001]. Additionally, a rise of >5 bpm in weekly average NHR from the preceding week was associated with 2.5 higher composite event risk (HR 2.51, 95% CI 1.22-5.18, P = 0.012) as well as total mortality (HR 11.21, 95% CI 3.55-35.37, P < 0.001) and cardiovascular hospitalization (HR 2.70, 95% CI 1.51-4.82, P <0.001).

Introduction

The past two decades have witnessed growing evidence for a strong relationship between high heart rate and adverse cardiovascular events.1,2 The resting heart rate has often been inferred from ECGs done randomly at different times during the day, which may be subject to some variability, thereby introducing some deviation from a true resting state.3 On the other hand, nocturnal heart rate (NHR) during sleep, being a state of true physiological rest, is likely to be a more accurate marker of resting heart rate, but has not been well studied as a prognostic risk marker. In addition, studies have traditionally relied on heart rate assessments at one fixed time point, often early in the course, to predict long-term outcomes.4 However, similar to other risk markers, dynamic measurements of heart rate changes over time could improve the specificity of this marker and also potentially allow timely, specific preemptive action. Using data from a large, nationwide cohort of patients with heart failure equipped with a wearable cardioverter-defibrillator (WCD) capable of recording all heart beats over the entire duration of use, we assessed whether dynamic monitoring of NHR allows timely identification of adverse cardiovascular events.

Methods

Study design and patient population

The WEARIT-France study (Clinical Trials.gov Identifier: NCT03319160) is a prospective nationwide cohort study assessing the use of the WCD in patients with heart failure across 88 French cardiology centres. The complete methodology has been described before.5 This study complies with the Declaration of Helsinki and an ethics committee approved the research protocol. All patients who agreed to participate were entered into the study after having given their informed consent.

The wearable cardioverter-defibrillator

The WCD technology used in the WEARIT-France study is a commercially available external defibrillator (LifeVest, ZOLL Cardiac Management Solutions, PA), guided by an algorithm to detect ventricular tachyarrhythmia events.6,7 The functioning of WCD has already been described8,9 and current indications are summarized in the last guidelines.10,11 During the index hospitalization when WCD therapy was initiated, the treating physician systematically assessed the appropriateness of WCD prescription and educated the patient regarding the transient risk for sudden cardiac death, functioning of the WCD, and benefits expected from the device. Additionally, just before discharge, a technical expert from the WCD company imparted 2 h of practical education to the patient, encompassing the nature of the disease, indication for WCD, alarm management, and remote transmission. The local remote monitoring team monitored daily wear duration on a regular basis.

Collected data and study end points

At the time of enrolment, medical history, comorbidities, symptoms, and other baseline characteristics were collected in addition to the indication for WCD. The WCD is equipped with four electrodes allowing the calculation of heart rate based on the R-R intervals, which is computed as the mean of all the heart rate provided every 5 min. The WCD prescription period is 90 days in France and the observation period was therefore limited to this period.

The telemonitoring platform also allows for heart rate monitoring. We specifically focused on NHR, which was defined as the mean heart rate from midnight to 7 a.m. because of lower heart rate variability (see Supplementary material online, Figure S1). In addition, we monitored NHR changes over time. To analyse mid-term predictive value of initial NHR trajectories over the WCD use period, patients were classified into four groups (High-Up, High-Down, Low-Up, Low-Down) based on NHR trajectory using a cut-off mean of 70bpm during the first seven nights of use (High if ≥70 bpm or Low if <70 bpm) and then according to an increase (Up) or decrease (Down) of NHR between Week 1 and Week 2 (see Supplementary material online, Figure S2). Similar to previous publications,1 mean cut-off of 70 bpm was determined because mean NHR was 68.8 (10.3) in this population. To assess the short-term dynamics of NHR, we defined ΔNHR as the difference between the weekly average NHR and that of the preceding week for each subject.

The primary endpoint was a composite of cardiovascular death and heart failure-related hospitalization. Endpoints were centrally adjudicated by an independent clinical events committee composed of three experts who adjudicated the events, by analysing the medical records/electrogram information, independent of each other and blinded to NHR and any additional information.

Statistical analysis

Preparation of this report was carried out in accordance with the STrengthening the Reporting of Observational studies in Epidemiology (STROBE) statement.12 Descriptive statistics were used to report major clinical characteristics and frequency of events. Continuous variables are presented as mean (standard deviation) or median and interquartile range (IQR) where appropriate and compared with Welch's t-test or Wilcoxon-Mann-Whitney test. Nominal variables were expressed as number and percentage and compared using the Pearson's χ2 test.

The time to event for each individual was defined from the first day of WCD wear to the day of first primary event, censoring, or end of followup (90 days), whichever came first. Cumulative incidence curves stratified by the initial NHR trajectories (High-Up, High-Down, Low-Up, Low-Down) were calculated by one minus the Kaplan-Meier estimator. Difference was assessed by the log-rank statistic. A multivariable Cox regression model with subjects' baseline characteristics as covariates was used to estimate the hazard ratios (HRs) of NHR initial trajectories for primary endpoint, adjusting on age, sex, body mass index, New York Heart Association class, left ventricular ejection fraction, prior hospitalization for heart failure, history of atrial fibrillation or kidney disease, and beta-locker se.

Nocturnal heart rate dynamics were plotted in the primary endpoint and he non-primary endpoint groups to evaluate changes in trajectory over ime. A nested case-control methodology was used matching 1 case for -9 controls to remove the effect of time since start of wear. In this way, n the day of an event (e.g. Day 14), a case is compared to a matched control f the same sex and on the same day of WCD wear.

To evaluate short-term dynamics of NHR (i.e. weekly changes in NHR), e used a prospective approach computing ΔNHR independently from the vent. ΔNHR was defined as the difference between the weekly average HR from 2 weeks back (W-2) and the week before (W-1) for each subject. since a previous publication showed a significant HR increase in the last 0 days before an adverse cardiovascular event,13 we assumed that a week represented adequate duration to reflect a clinically relevant change, but as not too long to miss any significant events. The ΔNHRs were updated weekly until the week just preceding the week of the event or until the end of observation period in case of no event (see Supplementary material online, Figure S3). ΔNHR was modelled as a continuous and a categorical covariate. The association between each endpoint and ΔNHR (W-1 minus W-2) as continuous covariates (per 5 bpm increase from W-2 to W-1) and categorical covariate (change > 5 bpm from W-2 to W-1) was assessed by the Cox model14,15 with adjustment for baseline NHR (2 weeks back: W-2) and for the confounders: age, sex, body mass index, New York Heart Association class, left ventricular ejection fraction, prior hospitalization for heart failure, history of atrial fibrillation or kidney disease, and beta-blocker use. The relationship between ΔNHR as a continuous variable and the HR for the primary endpoint was also examined by cubic spline curve with three knots, using a reference value of 60 bpm.

We performed sensitivity analysis among both genders, patients without history of atrial fibrillation and patients with New York Heart Association Class I and II, with respect to both mid-term NHR trajectories and short-term dynamics of NHR.

Analyses were performed using R software (version 4.1.3). All statistical tests performed were two-sided. A P-value of <0.05 was considered statistically significant. The proportional hazards assumption was tested and found satisfied.

Results

Baseline patient characteristics

Among 1157 patients enrolled in the WEARIT-France study, 1013 (88%) wore the WCD more than 2 weeks and were analysed. Clinical characteristics of the patients are listed in Table 1. The median age was 61 (IQR 53-68) years, 167 (16%) were females and median left ventricular ejection fraction was 26% (IQR 22-30). New York Heart Association status was Class I or II in 723 (71%) patients. A total of 76 (8%) patients had renal disease requiring therapy, 103 (10%) patients had history of atrial fibrillation, 71 (7%) had previous stroke. Regarding medical therapies, 905 (89%) patients were prescribed beta-blockers, 879 (87%) angiotensin-converting enzyme inhibitor or angiotensin receptor blocker, and 160 (16%) amiodarone.

Follow-up and primary endpoint evaluation

Median WCD wear time period was 68 (IQR 44-90) days in the overall patient population, with 58 (7%) patients experiencing events: 10 deaths (including 5 cardiovascular deaths) and 97 patients with cardiovascular hospitalizations (including 58 patients with heart failurerelated hospitalizations) (Table 2).

At the end of wear time period, 548 (54.1%) received an implantable cardioverterdefibrillator, and left ventricular ejection fraction improved in 343 (33.9%). When comparing patients with and without event, 49.5 vs. 53.8%, respectively had an implantable cardioverter defibrillator implantation and 4.1 vs. 35.9% had improved left ventricular ejection fraction (global P-value ≤ 0.001).

Initial nocturnal heart rate trajectories

Considering the whole population, the mean NHR was 68 •} 11 bpm during the first 2 weeks of WCD use and 64 +/-bpm during the last 2 weeks of WCD use (P < 0.001).

Looking at initial NHR trajectories classified into 4 groups (High-Up, High-Down, Low-Up, Low-Down), 123 (12%) patients were in the High-Up, 214 (22%) in the High-Down, 294 (30%) in the Low-Up, and 355 (36%) in the Low-Down group (27 patients were not classified because of missing NHR). The primary endpoint rate in the High-High group was significantly higher compared to the other groups (Log Rank P < 0.001; Figure 1). In multivariate Cox analysis, High-Up trajectory remained significantly associated with worse outcome [adjusted HR 6.08, 95% confidence interval (CI) 2.56-14.45; Low-Down as reference, P < 0.001] along with history of heart failure hospitalization (HR 2.26, 95% CI 1.25-4.10, P = 0.007), whereas angiotensin converting enzyme inhibitor/angiotensin receptor blocker use was associated with lower occurrence of primary endpoint (HR 0.30, 95% CI 0.16-0.57, P < 0.001). In sensitivity analyses, the risk associated with a High-Up trajectory was confirmed in both sexes, in patients without history of atrial fibrillation, and in patients with New York Heart Association Class I and II [HR 95% CI 6.09 (2.25-16.50), P < 0.001 for males, 8.85 (1.21-64.63), P = 0.032 for females; 5.58 (2.22-14.05), P < 0.001 for patients without atrial fibrillation, 8.75 (2.28-33.62), P = 0.002 for New York Heart Association Class I and II].

Short-term dynamics of nocturnal heart rate

Heart rate dynamics according to the primary endpoint are represented in Figure 2. In the group with cardiovascular events, NHR increased starting 7 to14 days before the event, whereas it decreased progressively in the group without primary endpoint. In the last 7 days before the event (from -8 to -1 day), NHR increased by +2 bpm (IQR -3; + 6) in the group with primary endpoint, whereas it decreased by -1 (IQR -1; -1) in the non-event group (Figure 2).

The distribution of ΔNHR is graphically shown in Supplementary material online, Figure S4. Most patients had only minimal change in NHR from the preceding week [median ΔNHR-0.44 bpm (IQR -2.44; 1.53)] during the WCD wear period. The restricted cubic spline model showed that increase in ΔNHR correlated linearly with higher risk for primary endpoint (Figure 3).

The association between individual components of the primary endpoint and NHR at the first night of WCD use as well as ΔNHR is summarized in Table 3. As a continuous covariate, both NHR at first night and ΔNHR were associated with total mortality and cardiovascular hospitalization. When considering ΔNHR as a continuous variable, each 5 bpm increase in ΔNHR was associated with a 41% higher risk of adverse events. This association was even stronger when considering ΔNHR as a categorical variable; an increase of ΔNHR > 5 bpm was associated with a 2.5-fold higher risk of primary endpoint (HR 2.51, 95% CI 1.22-5.18, P = 0.012), an 11-fold higher risk of death (HR 11.21, 95% CI 3.55-35.37, P < 0.001), and an almost three-fold higher risk of cardiovascular hospitalization (HR 2.70, 95% CI 1.51-4.82, P < 0.01). When considering subgroup analysis, similar results were obtained in males and females. In patients without history of atrial fibrillation, an increase of ΔNHR > 5 bpm was associated with two-fold higher risk of primary endpoint (HR 2.51; 95% CI 1.16-5.40, P = 0.019). Regarding patients with New York Heart Association Class I and II, an increase of NHR ≥ 5 as compared to the previous week was associated with point estimates for HR > 2 but was not significant due to loss of power.

Discussion

In this study, we found that NHR was associated with adverse cardiovascular events in a heart failure population within a short-to mid-term timeframe. Moreover, dynamic changes in NHR were demonstrated to correlate in a temporal fashion with hard clinical endpoints such as cardiovascular death and hospitalization for heart failure. Weekly rise of NHR > 5 bpm was especially associated with high risk, suggesting potentially actionable cut-offs for clinical application. While needing further validation in future studies, these observations suggest that NHR, especially with longitudinal monitoring, could be a valuable addition to the risk assessment arsenal for prediction of cardiovascular events.

Prediction of cardiovascular events

Because of the high rate of re-hospitalizations, high mortality, poor quality of life, and the substantial cost sustained by national healthcare systems, much effort has been made to identify the parameters/risk factors that can effectively contribute to prediction and prevention of decompensation events and hospitalizations in patients with heart failure. [START_REF] Gheorghiade | Systolic blood pressure at admission, clinical characteristics, and outcomes in patients hospitalized with acute heart failure[END_REF] Previous studies showed that heart rate was another parameter associated with adverse outcome in different settings, especially in heart failure.17 Nevertheless, prediction of cardiovascular events with 'static' heart rate assessment, reflecting one-time status, has limitations as it can be affected by a number of factors, potentially affecting specificity. In addition, event risk is a dynamic, time-varying phenomenon; therefore, it makes sense that continuous measurements would be preferable to a single one. [START_REF] Messerli | Risk factor variability and cardiovascular outcome: JACC review topic of the week[END_REF] In this regard, dynamic monitoring holds promise, wherein each patient serves as his/her own control and changes over time may yield higher sensitivity as well as specific risk assessment. Vazir et al.4 reported that, compared to the previous visit, an increase >5 bpm in resting heart rate was associated with 1.06 times higher risk of cardiovascular death or hospitalization for heart failure. However, HR assessment was irregular as it was evaluated at any time from every 2 weeks to every 4 months. A closer and automated measurement of heart rate over time, as in this study, could identify events with better accuracy and in a timely manner to avoid hospitalization or death. We assessed the mean NHR weekly and found that an increase of > 5 bpm was associated with a more than two-fold risk of cardiovascular death or hospitalization for heart failure.

Remote monitoring and connected devices

Telemedicine can allow for remote monitoring and management of patients with chronic cardiovascular diseases, making it possible to assess medication adherence and detecting early signs of decompensation before it results in additional complications or hospital readmission. Even though this has mostly been done with invasive devices,19 advances in technology now make it possible to use non-invasive solutions.20,21 Moreover, the large population of patients

Near-term prevention

Long-term risk prediction is often disappointing with imperfections in risk assessment s well as solutions (for instance, an implantable cardioverter defibrillator along with its side effects).25,26 Near-term prevention, which relies upon prompt action in response to warning signs, could allow timely intervention to avoid the adverse outcome but without the inconvenience of 'permanent' therapy.27 In fact, cardiovascular risk is dynamic and modulated by a variety of environmental factors, seasonal variations, and circadian rhythms. [START_REF] Arntz | Diurnal, weekly and seasonal variation of sudden death. Population-based analysis of 24,061 consecutive cases[END_REF] In the present study, we have demonstrated that dynamic monitoring of NHR has the potential to be not only a reliable predictor of cardiovascular events in patients with heart failure but could also pave the way towards near-term prevention of cardiovascular events.

Underlying mechanisms for heart rate increase before the event remain to be fully elucidated and compensatory tachycardia in response to volume overload could be one of the possible mechanisms. Nonetheless, the important point is that the NHR rise preceded major events such as hospitalization or death by a time period which appears reasonably sufficient for timely clinical intervention. One can imagine that in the future, combining clinical characteristics and remote monitoring will allow to identify specific groups at risk of coronary event or heart failure acutization or maybe sudden cardiac arrest. Therefore, we will be able to pre-empt these events and take specific measures.

Limitations

In this study, we have presented novel findings that may help refine use of heart rate as a marker to eventually improve short-term prediction and survival in heart failure populations; however, we need to acknowledge some limitations. First, this work should be viewed as a proof-of-concept study, as heart rate was collected using a WCD with limited follow-up, and may be not applicable to the entire heart failure population. However, as already mentioned, similar information is obtainable for other implanted devices and wearable sensors; as a result, this approach can be further tested and expanded in the future. Our study population consisted mainly in patients with ischaemic cardiomyopathy, so caution has to be exercised in extrapolating results to other causes of heart failure. Moreover, a high proportion of the patients were on beta-blockers; whether the magnitude of risk associations would be different in patients not on heart rate modulating drugs warrants further evaluation. Finally, further work is needed to confirm the associations reported in this study as well as the effectiveness of a strategy based on NHR monitoring to reduce hospitalization and mortality.

Conclusions

In patients with WCD, continuous dynamic monitoring of NHR helps to predict adverse cardiovascular events. It holds promise as a means of improving risk prediction prior to timely pre-emptive action, enabling reduction of adverse outcomes in populations at risk.

with implanted and wearable devices (such as implantable cardioverter-defibrillator, cardiac resynchronization therapy and WCD) with rapid expansion of remote monitoring technology presents an important opportunity, which needs to be leveraged to improve risk prediction. To the best of our knowledge, this work is the first to use heart rate collected in an automatic and continuous manner. Indeed, in prior work attempting to predict cardiovascular events, heart rate was measured manually or derived from a single 12 lead ECG.4,13,22 While we studied a selected population of patients equipped with a WCD, signal acquisition is becoming easier with the recent development of a wide range of connected devices,23 which have become deeply entrenched in our daily lives. Despite the promise of remote patient monitoring, this technology has thus far remained relatively underutilized. In the era of artificial intelligence, remote and increasingly personalized patient care, one can imagine that heart rate could be monitored with a simple connected watch, greatly expanding the applicability of this concept.24 With such connected devices, continuous data acquisition has the potential to open up avenues for near-term prevention, where dynamic changes in monitored parameters can be used to take corrective, 'pre-emptive' action, avoiding adverse events.

Acknowledgements

We deeply thank all the investigators of the WEARIT-France Study: Laurence Guedon Moreau, Mohanad Mahfoud, Philippe Ritter, Jean Marc Dupuis, Arab Yalioua, Antoine Dompnier, Marc Goralski, Saïda Cheggour, Christophe Leclercq, Eloi Marijon, Jérôme Bouet, Clémentine André, Fabrice Extramiana, Florent Briand, Anne Rollin, Gilles Lande, Philippe Chevalier, Jean-Luc Pasquié, Nicolas Lellouche, Bruno Degand, Yves Cottin, François Jourda, Jacques Mansourati, Angeline Martin, Antoine Da Costa, Olivier Billon, Isabelle Cheradame, Jean-Sylvain Hermida, Julien Bayard, Romain Eschalier, Benoit Guy Moyat, Vladimir Manenti, Hervé Gorka, Nicolas Combes, Antoine Milhem, Olivier Piot, Yves Guyomar, Renaud Fouché, Elisabeth Somody Litoux, Anne Quentin, Jean Baptiste Berneau, Thierry Tibi, Aurélien Miralles, Fiorella Salerno, Peggy Jacon, Laurentin Nitu, Damien Brunet, Hugues Bader, Arnaud Savoure, Pascal Sagnol, Frédéric Sebag, Sebastien Buffler, Joël Fedida, Julien Pineau, Guillaume De Geeter, Adlane Zemmoura, Ahmed Salhi, Eric Verbrugge, Sophie Pynn, Jean Pierre Gueffet, Walid Amara, Aurélien Seemann, Pierre Winum, Nicolas Johnson, Claire Vanesson, Alexis Mechulan, Vincent Hugon, Xavier Marchand, Jean-Claude Deharo, Sylvain Reuter, Raphael Sandras, Damien Legalloi, Olivier Garrier, Gaël Jauvert, Jérémy Descoux, Hugues Blangy, Alain Lebon, Pierre Sultan, Franck Sibellas, Samer Mousi, Gabriel Laurent, Maxime Pons, Halim Marzak, Jérôme Clerc, Hassan Barake, Aurélie Guiot, Vincent Algalarrondo, Laurent Palud, Denis Raguin, Frédéric Treguer, Pierre Le Franc, Isabelle Lecardonnel, Frederic Fossati, Ghassan Moubarak, Omar Bilel Mokrani, Pascal Chavernac. We gratefully thank Nobutaka Murata and Nicole Bianco of ZOLL for providing the technical assistance for data analysis.

Funding

This work was supported by the Institut National de la Santé et de la Recherche Médicale (INSERM) and Zoll Medical Corporation.

Data availability

The data underlying this article will be shared on reasonable request to ZOLL.

Conflict of interest:

E.M. is consultant for ZOLL, Boston, Medtronic and received unrestricted grants from industrial partners (Medtronic, Abbott, Boston Scientific, Microport, Biotronik and ZOLL). R.G. have been compensated by ZOLL for giving presentations. The remaining authors have nothing to disclose.