Irene Camps
email: icamps@ucm.es

Pedro A González-Calero
email: pedrop@ucm.es

María Angeles Quiroga

Pedro P Gómez-Martín

Measuring preferences in game mechanics: towards personalized chocolate-covered broccoli

Keywords: Serious games, Informal learning, Games in museums, Games for education, Augmented Reality

When developing educational games we face the problem of finding the right design for making the learning activities as intrinsic to the game mechanics as possible. Nevertheless, in many cases it is not possible to fully integrate the learning content into the game play, resulting in the well known "chocolate-covered broccoli" game design. The long term goal of the work presented here is to determine whether a personalized selection of game mechanics for the playful part, the game mechanics around the learning part of the game, can improve the satisfaction of the player and therefore make the whole learning experience more enjoyable. The first step towards that goal is to obtain a model for the preferences of game mechanics for a particular type of game, and later use that model to guide the selection of game mechanics. In this paper, we present Enigma MNCN a treasure hunt for mobile devices designed for the National Museum of Natural Sciences of Spain and some experimental results intended to identify preferences for game mechanics in that type of game across demographic variables. The main finding of these experiments is that preferences in game mechanics get shadowed when combined with a mostly disliked learning mechanic.

Introduction

The current challenge for museums is how to successfully turn their institutional knowledge and authority into meaningful, engaging experiences by leveraging the appropriate technological media in the context of their physical settings, and for heterogeneous audiences [START_REF] Roussou | The museum as digital storyteller: Collaborative participatory creation of interactive digital experiences[END_REF]. In order to solve this problem, a growing number of initiatives integrating serious games, gamification, augmented reality and virtual reality through mobile devices have appeared in the last years [START_REF] Paliokas | The use of serious games in museum visits and exhibitions: A systematic mapping study[END_REF][START_REF] Koutsabasis | Empirical Evaluations of Interactive Systems in Cultural Heritage: A Review[END_REF].

In this paper we present results from a project that intends to promote informal learning in a natural history museum through a treasure hunt type of game. The game incorporates image recognition, mini-games, 3D virtual reconstructions and augmented reality elements, being part of a growing number of initiatives that seek to exploit the use of augmented reality and related technologies in informal science learning sites [START_REF] Goff | Applications of augmented reality in informal science learning sites: a review[END_REF].

As designers of educational games we face the problem of designing just a new version of the well known "chocolate-covered broccoli", a term first introduced by Amy Bruckman in a presentation at the Developers Game Conference in 1999 [START_REF] Bruckman | Can educational be fun?[END_REF]. Actually, she used the term "dipped-covered broccoli" for an approach used when combining learning content with gameplay where the gaming element of the product is used as a separate reward or sugar-coating for completing the educational content. This is an intrinsic problem of educational games. Although digital games may be capable of providing activities which are intrinsically motivating in their own right, it is critical to consider the effect of adding learning content to an intrinsically motivating game [START_REF] Habgood | Motivating children to learn effectively: Exploring the value of intrinsic integration in educational games[END_REF].

Ideally the learning goals in an educational game should be attained through activities that are intrinsic to the game play. For certain subjects or learning goals the use of intrinsic game mechanics with learning purposes can be straightforward, for example when the game serves as some kind of simulator of the target content. On the other hand, some types of learning content are very hard to turn into game mechanics, such as factual knowledge, as the one we want to include in our game for a Natural History museum. Therefore, either we accept that games are just adequate for certain types of learning content or we accept some broccoli in our game recipes.

Enigma MNCN is a treasure hunt for mobile devices designed for the National Museum of Natural Sciences (referred to by its Spanish acronym MNCN) in Madrid, one of the oldest museums of Natural History in Europe and the most important in Spain. This is the second game in the Enigma saga1 , after Enigma Galdiano released in 2016 and designed to be played at the Lázaro Galdiano Museum in Madrid [START_REF] Camps-Ortueta | Combining augmented reality with real maps to promote social interaction in treasure hunts[END_REF].

Enigma MNCN, described in more detail in Section 2, is designed for kids from 8 to 12 years old. The kid plays as a Paleontologist apprentice who has to find some objects in the collection and solve some puzzles and quizzes along the way. The learning content in the game is provided through a field notebook with images and textual information about pieces in the museum, which pages are revealed as goals in the game are fulfilled. In order to make the reading of the field notebook more intrinsic to the game, we include quizzes which answer is included in one of the pages of the notebook, usually the last one that was revealed.

Enigma MNCN includes a number of mini-games and treasure hunt mechanics, the chocolate, in order to cover a reading task, the broccoli. The long term goal of the work presented here is to determine whether a personalized version of the playful part, the game mechanics around the learning part of the game, can improve the satisfaction of the player and therefore make the whole learning experience more enjoyable. Keeping up with the metaphor, we want to determine whether a personalized chocolate recipe can make the chocolate-covered broccoli taste better. The first step towards that goal is to obtain a model for the preferences of game mechanics for this particular type of game, so that we can later use that model to guide the selection of game mechanics.

In this paper we present the different game mechanics included in Enigma MNCN and the results of several experiments trying to determine the preferences for those game mechanics among kids from 8 to 12, based on age and gender. We run experiments with two different versions of the game, the full version including the learning component (field notebook), and a reduced one without the learning component, in order to measure the variability in preferences for game mechanics in a treasure hunt game versus an educational version of the game. The main finding of these initial experiments is that preferences in game mechanics get shadowed when combined with a mostly disliked learning mechanic. We can detect significant differences in preferences for game mechanics when evaluating those preferences in the purely playful version of the game, but those differences get blurred when measured in the educational version, with broccoli (i.e., reading tasks) added in.

The rest of the paper runs as follows. Next Section describes the game mechanics in Enigma MNCN. Section 3 details the experimental set-up along with the results from the experiments and our conclusions about those results. Finally, Section 4 presents related work and concludes the paper.

Enigma MNCN

Enigma MNCN is a treasure hunt for mobile devices designed to be played at the National Museum of Natural Sciences of Spain, in Madrid. The players, kids between 8 and 12 years, are committed to become the new assistant of Dr. Anning, one of the paleontologist in the museum, helped by two of her current assistants: Pérez and Neand. The three of them, depicted in Figure 1 will alternatively propose new challenges that will let the kid to demonstrate her merits to join the team. The theme of the exposition where the game is played is the evolution of life on Earth, from the first micro-organisms to Homo Sapiens, through a collection of fossils, skeletons, reconstructions and illustrations that recreate the life on Earth at different points in time.

The core game mechanic in Enigma uses the camera of a mobile device to recognize an object in the museum (we use Vuforia T M and Unity 3D T M as underlying technology). The object to be found is indicated to the player with its scientific name, such as Brachiopoda Strophonema, Calamopora Spongites or Ammonitida Ammonitina Perisphinctidae. We want the kids to pay attention to the signs in the exposition and make sense of the organization of the objects, where, for example, every fossil from the Ammonitida family is in the same showcase. Once the name of the object to be found has been read, the mobile turns into "search mode" becoming the "Paleo lens" in the game, as depicted in Figure 2.

In some search tasks of the game just one object has to be found while in others we provide up to three different names of objects that have to be found in any order. Depending on the complexity of the search we can also provide a hollow silhouette of the target object in order to facilitate its identification. Since the player will typically forget the exact name of the object, she can ask Pérez, our friendly sloth, to remind her the name (see the sloth in the right bottom of Figure 2).

As an additional clue, we can also provide the time depicted in the showcase of the object to be found. The exposition uses the well known metaphor of mapping the history of Earth into 24 hours, so for example first fossils appear at 5:36 am and humans at 11:58 pm. Once the camera is pointing at the target object, it will be recognized and the task will have been fulfilled. We do not used QR codes but the actual objects in the collection. Since kids are usually unfamiliar with this technology, the game begins with a tutorial explaining: how the Paleo lens can be used to recognize objects in the museum; that you can ask Pérez to remind you the name of the object; and that it is possible to give up and quit a search if you can not make the Paleo lens to see it (kids almost never give up in their searches). Every mini-game comes after a Paleo lens search and usually relates somehow to the object just found. This serve to give some context for the mini-game, since we know where is the kid in the museum and we can use in the mini-game those elements in front of her.

Packaging, Figure 3 left, is a puzzle game where the kid has to put all the pieces appearing to the right of the box, inside of the box. Following the narrative of the game, the kid is helping the museum by packaging some fossils that need to be sent to another museum. The fossils in the mini-game are similar to those in the showcase in front of the kid.

Skeletons, as Packaging, also require visual-spatial skills to be solved. As shown in Figure 3 right, a partial skeleton is provided on the left with some missing bones on the right. The goal is to place the bones at the right positions. To provide and contextualize the task, the game has taken the kid in front of that same skeleton in the museum (a Deinotherium in the example of Figure 3), so that she can look at the original to get inspiration. Magic fields are 3D reconstructions of prehistoric life. The Museum already displays large panels with illustrations of prehistoric life (an example is depicted in Figure 4 left). A magic field is a 3D scene that is loaded after a search task that has led the player in front of the illustration panel, making that illustration come alive. The goal in the mini-game is to move around the 3D scene, through the gyroscope of the mobile device, and find a particular prehistoric animal, which usually corresponds to an skeleton we have seen before in the game.

AR hunts use augmented reality technology to insert an image of a prehistoric animal into one of the illustration panels of the museum, as shown in Figure 4 left where a Meganeura, an extinct insect from the Carboniferous period, is moving around the illustration as seen through the camera of the device. The goal is to capture the moving animal by tapping it.

For the educational version of the game we add two more elements: the field notebook and the quizzes. The field notebook plays the role of the notebook of Dr Anning, where the paleontologist is writing down some of the main facts about the pieces she find, which are actually the ones that we found in the game. After every search, one more page, as the one showed in Figure 5 left, is added to the initially empty notebook. In order to motivate the kids to read the contents of the notebook we include a new game mechanic: the quizzes.

The quizzes, as the one showed in Figure 5 right, are multiple choice questions with a humorous tone where there is only one right answer. There is time limit to answer the question of 30 seconds, but the kid can stop the timer by opening the field notebook and reading it. Every time, the right answer to the quiz is provided in the notebook. The first version of the game used for the experiments consists of 8 stages, where each stage includes a search or multi-search and a mini-game, roughly including two mini-games of each type. In average it takes between 20 and 25 minutes to complete this version of the game. The educational version includes, in addition, 4 quizzes, every 2 or 3 stages, and adds between 7 and 8 more minutes to the gameplay time.

Experiments and Results

Experimental set-up

The experiments are run in the museum, when it is closed to the general public. In every run of the experiment we have a group of between 10 and 14 kids playing the game individually, using the same type of device, a Lenovo T M TAB3 10 Plus, provided by the museum. The choice of the size of the group seeks to find a balance between the effort to run the experiments and the bias from having more people playing the game at the same time. Imagine that if you see five kids pointing their devices at the same point then maybe you should be pointing there too. In our experience, the variability in the time that the kids need to find the first objects alleviates this bias, and they quickly distribute among the different searches.

In the game we collect metrics obtained during game play, including data such as the time spent in every task, whether the task is successful or not, and how many times did the player make use of the help. Although the game is implemented in Unity 3D which offer some functionality for metrics collection, we have developed our own system that collects the metrics of interest and send them, at the end of every task in the game, as JSON files to a server where they are made persistent. The opinion of the kids about the game play mechanics are collected at the end of the game, and also made persistent as metrics in the server for later analysis.

For the experiments described in this paper, satisfaction is our main variable. We collect satisfaction data through a questionnaire integrated at the end of the game. On the questionnaire there is a question for every mechanic included in the game, with an image from the game to remind the kid what mechanic is she being asked for (see Figure 6). Answers are given by selecting among a smiley, a neutral or a sad face, to make it more appealing for kids. Since most of the answers were smiley faces, we decided to dichotomize the answers into "positive" (smiley face) and "negative" (sad or neutral face), in order to increase the power in our analysis. We ran two sets of experiments. The first one with the treasure hunt game without learning mechanics (quizzes and field notebook), and the second one with the full version of the educational game.

First experiment: treasure hunt game

The sample of the first experiment consisted of 30 subjects with an average age of 9.4 years (SD=0,56;17 subjects of 9 years and 13 of 10 years). Of the 30 subjects, 14 were boys and 16 girls. We consider it a specially homogeneous sample since they all attended fourth grade at the same school. Due to the small size of the sample on this experiment, we decided to run only non-parametric analysis.

Average satisfaction was above 80%, an encouraging result for the game, but our main goal was to determine whether we can detect differences in preferences for the different mechanics. For this purpose we have analyzed differences in preferences based on gender, Figure 7, and age, Figure 8. Although results are not statistically significant (p values higher than .05 for each chi square analysis run), we can observe that boys have a larger preference than girls for the Paleo lens, packaging and skeletons mechanics, while girls tend to prefer AR hunt and magic fields. Considering the type of activities in the game mechanics we can conclude that boys prefer instrumental activities, focused on action, while girls tend to prefer expressive activities, focused on aesthetics and emotion. It is interesting to see this tendency, usually observed in adults [START_REF] Sonja | Instrumentality and expressiveness at work[END_REF], to appear in young kids.

Regarding age, even with such a little age difference, we can observe some differences between 9 and 10 year old kids, as shown in not statistically significant results point in the direction that 9 year old kids find more enjoyable less complex mechanics, such as Paleo lens and magic fields, while 10 year old kids tend to prefer skeletons or packaging that require a more complex problem solving abilities. These initial results make us think in the possibility of finding some correlation between game mechanics preferences and demographic data such as age and gender.

Second experiment: educational treasure hunt

A total of 213 children participated in the second experiment. A part of them did not finish the game (N = 28, which is 13%), and from the remaining 185 an additional 20% (N = 36) were outside of the established age range. Therefore, the results refer to a sample of 149 subjects, in the age range from 8 to 12 (M = 9.60, SD = 1.17). Of these 73 were boys and 76 were girls.

Average satisfaction is again above 80%, as shown in Figure 9, except for the additional learning mechanic: quizzes. Regarding differences based on gender we can observe in Figure 10 that differences are at most of 3 percentage points, or 4 in the case of quizzes, what does not introduce any significant difference.

Higher variability can be observed when considering age differences, as shown in Figure 11. Nevertheless, again the differences are not statistically significant, as shown by the results of Pearson's chi-squared test (χ 2 ) in Table 1.

Our hypothesis is that although differences in preferences for game mechanics can be observed just by considering demographics data as study 1 shows, such differences mostly disappear when adding a mechanic that has a much lower acceptance value, and therefore make the rest of mechanics equally acceptable in comparison.

Related work and conclusions

Research on personalized content for serious games is a growing area of interest.

Once it has been accepted that digital games are an appropriate instrument for applications beyond pure entertainment, in training and communication, the question of how to personalize serious games is being raised to increase their effectiveness. In this sense, we find works that show that the effectiveness of serious games improves with personalization in adults, and others that advance in the definition of instruments to facilitate such customization. In [START_REF] Orji | Improving the efficacy of games for change using personalization models[END_REF] some initial results are provided showing the importance of tailoring games for change in the context of a game designed to improve healthy eating habits. Tailoring the game design to players' personality type improved the effectiveness of the game, as was later shown in [START_REF] Orji | Personalizing persuasive strategies in gameful systems to gamification user types[END_REF] with a large-scale study of more than 500 participants where their results reveal that people's gamification user Regarding related work on instruments designed to facilitate the constructions of customized games, in [START_REF] Tondello | A framework and taxonomy of videogame playing preferences[END_REF] a conceptual framework of player preferences based on game elements and game playing styles is presented. Such framework can be used by designers to create games that are tailored to their target audience. Applying these ideas to serious games, [START_REF] Tondello | Recommender systems for personalized gamification[END_REF] presents a general framework for personalized gameful applications using recommender systems, by describing the different building blocks of a recommender system (users, items, and transactions) in a personalized gamification context.

In this paper we have presented some initial results of experiments conducted in order to determine whether a model for game mechanics preferences can be found for an specific type of games: treasure hunts in museums with educational purposes for children. We have experimentally analyzed differences in preferences based on demographics, which are easier to measure than other potentially more informative differences such as temperament or cognitive capacities that we plan to measure in the future. Accepting that in some situations it is not possible to find an intrinsic game mechanic that serve the learning purposes of an educational game, our goal is to find if by tailoring the selection of accompanying game mechanics we can improve the whole educational experience.

For this purpose, we first ran a set of experiments with a non educational version of the game, and obtained initial values of variability in preferences along age and gender axes. Although not statistically significant, we found some indication of the existence of a preference model. game. In this case differences in preference for game mechanics were hardly measurable, invalidating our initial results. Our hypothesis is that although differences in preferences for game mechanics can be observed just by considering demographics data, such differences mostly disappear when adding a mechanic that has a much lower acceptance value, and therefore make the rest of mechanics equally acceptable in comparison. This effect resembles the "contrast effect" from Psychology, or the "contrast principle" as defined by [START_REF] Cialdini | Influence: The Psychology of Persuasion, Revised Edition[END_REF]. We need to collect additional data in order to test this hypothesis, and we will do in future work.

Also as future work, we want to measure the effect of the time spent with the educational content, in our case reading the field notebook in the game, both in terms of the quality of the answers for the quizzes in the game, and for the general satisfaction of the player. It would be a positive result if we could demonstrate that making use of the educational content in the game promotes learning, and does not decrease the quality of the playful experience.

Fig. 1 .

 1 Fig. 1. Enigma MNCN characters

Fig. 2 .

 2 Fig. 2. Paleo lens while searching

Fig. 3 .

 3 Fig. 3. Packaging and skeletons

Fig. 4 .

 4 Fig. 4. AR hunt and Magic field

Fig. 5 .

 5 Fig. 5. Field notebook and quiz

Fig. 6 .

 6 Fig. 6. Satisfaction questionnaire

Fig. 7 .

 7 Fig. 7. First experiment: differences by gender

FigureFig. 8 .

 8 Fig. 8. First experiment: differences by age

Fig. 10 .

 10 Fig. 10. Second experiment: differences by gender

Fig. 11 .

 11 Fig. 11. Second experiment: differences by age

Table 1 .

 1 Pearson's chi-squared test (χ 2 ) on age differences

							Positive Negative
		100
		80		87	85	82	81	86
	Scores	60					55
		40
		20
		0	Paleo lens Packaging Skeleton	AR Hunt Magic field Quizzes
				Fig. 9. Satisfaction by game mechanic
				8y	9y	10y	11-12y	χ 2	p
				pos neg pos neg pos neg pos neg
	Paleo lens 10 3 33 2 43 9 23 2 4.26	.23
	Packaging 11 2 29 6 45 7 21 4 0.24	.97
	Skeleton		9 4 31 4 43 9 21 4 2.56	.46
	AR Hunt	10 3 31 4 41 11 20 5 1.63	.65
	Magic field 10 3 31 3 44 8 23 2 2.52	.47
	Quizzes		4 9 20 15 29 23 16 9 3.98	.26

http://www.padaonegames.com/enigma/

Acknowledgments

We are very grateful to the personnel at the National Museum of Natural Sciences in Madrid for their support and assistance in designing and evaluating Enigma Ciencia, in particular to Luis Barrera and Pilar López. We also thank the anonymous reviewers for their insightful comments and suggestions.

This work is partly supported by the Spanish Ministry of Economy, Industry and Competitiveness (TIN2017-87330-R).


Valentina Nisi
email: valentina.nisi@gmail.com

Vanessa Cesario
email: vanessa.cesario@m-iti.org

Nuno Nunes
email: nunojnunes@tecnico.ulisboa.pt

Augmented Reality Museum's Gaming for Digital Natives: Haunted Encounters in the Carvalhal's Palace

Keywords: Museums and cultural heritage, digital natives, gaming, augmented reality, interactive storytelling

. In this article, we present the description and rational behind Memories of Carvalhal's Palace: Haunted Encounters mobile gaming application and then discuss the results of first empirical tests performed to evaluate the usefulness and usability of the game.

Introduction

As museums move away from being places where exhibits are collected and displayed, they become spaces where people can actively engage in personalized discoveries and challenges [START_REF] Falk | Learning from Museums: Visitor Experiences and the Making of Meaning[END_REF]. Traditionally, support for museum visits was limited to audio guides and interactive kiosks. However, the fast and wide uptake of interactive and mobile technologies is making audiences more eager to engage in active experiences. Museums are trying to keep up this challenge and enhancing their context to provide more engaging opportunities to interact with the exhibits secured behind glass shelves [START_REF] Simon | The Participatory Museum[END_REF].

Many studies underline the importance of providing museums with improved user experiences both at individual and cooperative levels [START_REF] Ghiani | UbiCicero: A location-aware, multidevice museum guide[END_REF]. Games and narrative elements are proven to ameliorate engagement, motivation and learning within edutainment environments [START_REF] Jemmali | Educational Game Design: An Empirical Study of the Effects of Narrative[END_REF]. Moreover, Falk reports on museums experiences that "one size does not fits all" [START_REF] Falk | Identity and the Museum Visitor Experience[END_REF], in particular when addressing the "digital natives" generation (currently 15-19 years old), whose beliefs and behaviors are quite different from their previous generations [START_REF] Prensky | Digital Natives, Digital Immigrants Part 1[END_REF]. While museums often offer guided tours for children and adults, very little is designed for the digital natives [START_REF] Tzibazi | Participatory Action Research with young people in museums[END_REF].

Here we present Memories of Carvalhal's Palace -Haunted Encounters (MoCP-HE), a location-based game which involves players in uncovering factual and fictional elements of a museum premises and collection. We designed MoCP-HE as treasure hunt experience which makes use of AR in order to engage digital native teenagers in pursuing scientific knowledge about the museum exhibits. The goal of this project is to use gaming and narrative elements to influence our target group engagement with the museum content. The experience is currently being studied in order to understand teenage dynamics and preferences in museums providing a way to analyze the complex relationship between context of use, technological solutions, engagement and learning effectiveness. In the rest of this paper we first discuss related work, including museums related location-based games and stories. Then we describe the game mechanics and technological platforms used in Memories of Carvalhal's Palace: Haunted Encounters. Lastly, we report on some first preliminary user testing carried out with young gamers and users and provide some concluding remarks and indications for future work. The evolution of smartphones and tablets with technologies like GPS, gyroscopes, accelerometers and AR enables the development of a new generation of games with great potential to be applied in location-based experiences [START_REF] Fraser | Assembling History[END_REF]. For instance, gamified tour experiences can be tailored to visitors by calculating their position and orientations.

Referred to Long et al. [START_REF] Long | Cyberguide: Prototyping Context-aware Mobile Applications[END_REF], as the position-aware handheld intelligent tour, these mobile apps are capable of orienting visitors around public spaces and provide them with custom-fit information [START_REF] Long | Cyberguide: Prototyping Context-aware Mobile Applications[END_REF]. Support and gamified elements of the tour can be accomplished and delivered by interpreting the user behaviour; physical movement within the space and how they interact with the object or artefact of an exhibit, a monument, or another element of interest [START_REF] Petrelli | HyperAudio: Location-awareness + Adaptivity[END_REF]. The user's information combined with HCI concepts and location-aware systems can augment the engagement of the user, support their everyday activity and gain more effectiveness in the mobile delivery of information (links, audio messages, images, videos, 3D models) [START_REF] Petrelli | HyperAudio: Location-awareness + Adaptivity[END_REF]. Exploration and enhancement of environments with 3D and 360VR modelling, allow the exploration of reconstructions of public spaces. Examples include, Mehringplatz neighbourhood in Berlin [START_REF] Nisi | Yasmine's Adventures: An Interactive Urban Experience Exploring the Sociocultural Potential of Digital Entertainment[END_REF] or the convent of São Francisco in Funchal [START_REF] Dionisio | Fragments of Laura: Incorporating Mobile Virtual Reality in Location Aware Mobile Storytelling Experiences[END_REF] helping not only in the narrative of the stories but also facilitating acquisition of knowledge in regards to the physical space. In addition, many experiences are designed to work using different platforms that complement the participant interaction within the physical space, making connections between displays, context-aware, storytelling, and historical events [START_REF] Fraser | Assembling History[END_REF]. Recently several research projects developed around context-aware experiences and personalising edutainment in cultural heritage grounds. Personal Experiences with Active Cultural Heritage (PEACH), aims at developing educational and entertaining experiences (edutainment) tailored to the user background, needs and interest of the audience [START_REF] Stock | PEACH -Intelligent Interfaces for Museum Visits[END_REF]. Immersive experiences like 360º Mobile Reality (MVR) are more accessible thanks to the devices affordance and advancement in mobile technology. The rapid growth of these technologies creates new research and design opportunities widely desirable and enjoyable by a wide range of users [START_REF] Stock | PEACH -Intelligent Interfaces for Museum Visits[END_REF]. Nevertheless, in order to be widely adopted in public settings we need to overcome several design and technical challenges [START_REF] Dionisio | Fragments of Laura: Incorporating Mobile Virtual Reality in Location Aware Mobile Storytelling Experiences[END_REF]. As a consequence several guidelines for the development of interactive context-aware systems in physical spaces, especially, those devoted to tourism and cultural heritage have been identified: 1) flexibility to enable visitors to explore and learn in their own way, controlling their own pace; 2) content-aware information tailor to their context (personal and environmental); 3) Support for dynamic information -aware of changes in venues, times, menus, opening and closing hours of events, need always to be updated; and 4) Remote access to interactive (online) services [START_REF] Cheverst | Experiences of Developing and Deploying a Context-aware Tourist Guide: The GUIDE Project[END_REF].

Serious Games in the Cultural Sector

Several projects in the area of serious games in the cultural sector are supported by museums. Examples include, Yong's China Quest Adventure [START_REF]Yong's China Quest Adventure[END_REF]; The China Game [START_REF]The China Game[END_REF] about Chinese traditions; Fascinating Egyptian Mummies about the spiritual beliefs of the ancient Egyptians; The Great Bible Race [START_REF]Great Bible Race[END_REF] dealing with the religious roots of the Western civilization; and, the Mosaica project [START_REF]6th EU Framework Program-IST-034984 MOSAICA-Semantically enhanced, multifaceted, collaborative access to cultural heritage[END_REF] developed a Jewish heritage game. In the past few decades, a variety of game-based applications were designed for different media platforms and visitor types [START_REF] Pierroux | MUSTEL: Framing the Design of Technology-Enhanced Learning Activities for Museum Visitors[END_REF]. Those games are frequently entailed with the goal of using mobile devices to guide families' explorations through solving mystery and treasure hunting [START_REF] Cabrera | Mystery in the Museum: Collaborative Learning Activities Using Handheld Devices[END_REF], [START_REF] Dini | An Environment to Support Multi-user Interaction and Cooperation for Improving Museum Visits Through Games[END_REF]. Cabrera and colleagues [START_REF] Cabrera | Mystery in the Museum: Collaborative Learning Activities Using Handheld Devices[END_REF] built an interactive museum guide called Mystery in the Museum, deployed at historical/cultural museums, which allowed students to play and perform tasks related to certain artifacts while stimulating their imagination. The game mechanics involves exhibits related puzzles such as scrambled images of specific exhibits and verses from manuscripts of the museum. The study revealed that some of the users (13-19 years old teenagers) lost interest in the interactive guide due to the complexity of the tasks, while others switched the focus from the displayed artifacts to the handheld computers.

Ghost Detector [START_REF] Nilsson | Ghosts! A Location-Based Bluetooth LE Mobile Game for Museum Exploration[END_REF] is a story-driven location-based museum game for children. In this game, ghosts of various museum artefacts appear on the screen of the visitors' mobile device and challenges them to find the artefacts that the ghosts are representing. While evaluating this game, children were observed running through the corridors, paying attention to the feedback on the smartphone as well as the artefacts surrounding them. This study highlights how the introduction of the ubiquitous game undoubtedly influenced that level of excitement and engagement with the museum premises.

Intrigue at the museum [START_REF] Xhembulla | Intrigue at the Museum: Facilitating Engagement and Learning through a Location-Based Mobile Game[END_REF] is a plot-driven mobile game for children structured around exploration and tasks performance. It is a single-player game, and its plot invites visitors to search for a thief in the museum among a set of virtual characters. Clues are given to the players and as they solve riddles after scanning tags deployed in the building. Following a constructivist approach, the game allows children to explore the museum environment freely, according to their interests and agenda. Moreover, AR gaming and storytelling and cultural sector experiments are proliferous and date back to the beginning of the new millennia [25][26] [ref]. From the related work reported, location-based mobile games seem to represent a relevant learning tool and offer potential for further research and improvements.

Gaming as a strategy to Involve Teens in Museum Experiences

While gaming and narrative as strategies to engage young audiences has been extensively studied, the particular application of these techniques to engage teenage public in museum is somehow still in development. Teenagers and participatory design within museum studies are covered by some papers in the Interaction Design Children (IDC) Community. For instance, the study Digital Natives [START_REF] Iversen | Scandinavian Participatory Design: Dialogic Curation with Teenagers[END_REF] where teens (currently 15-19 years old) collaborated with designers, programmers, anthropologists and museum curators to create four digital installations for an exhibition. The case study Gaming the Museum [START_REF] Dindler | Participatory Design at the Museum: Inquiring into Children's Everyday Engagement in Cultural Heritage[END_REF] is another example that started from everyday practices where children's (14-15 years old) everyday engagement was strong and thus computers games and online communities were chosen to start a process of creating a game for a museum.

According to formal studies about teenage preferences regarding museum engagement, Cesário et al. [START_REF] Cesário | Analysing Texts and Drawings: The Teenage Perspective on Enjoyable Museum Experiences[END_REF], [START_REF] Cesário | Design Patterns to Enhance Teens' Museum Experiences[END_REF] identified that mobile experience for museum's teenage audiences should first and foremost include 1) Gaming and storytelling aspects, secondarily teens look for 2) Interaction elements, 3) and Social media connections and last but not least 4) Museum and exhibits relevant information. Based on these findings and in accordance with the museum goals, the authors in collaboration with a team of creatives and developers, designed Memories of Carvalhal's Palace -Haunted Encounters, an interactive mobile game experience, engaging teens in solving a mystery thorough exploring the museum and its embalmed species collection.

The Mobile Game Experience

The Memories of Carvalhal's Palace -Haunted Encounters is a location-specific nonlinear game deployed at the Natural History Museum of Funchal (NHMF). The museum was once the residence of a noble Madeiran family, the Carvalhal's, before being donated to the city municipality in order to be transformed into a museum. The game builds on the historical backstory of the building enhanced by the embalmed species.

The game suggests the premises are haunted by mysterious forces which are disturbing the status quo. At the beginning of the game, two characters ask the player to help them solve the mystery behind the upsetting circumstances in which the museum verses (Fig. 3). Through a game of shadows, the audience is challenged to find and interact with the exhibits displaced around the museum as they match the shadow presented to them on the screen (Fig. 2). By finding the correct animal, the players unlock an AR 3D model of it (Fig. 4, left side) and are prompted with a multiple answers quiz style question that require them to closely check the species in display. As the players progress in their quest, they are rewarded with pieces of a map that will lead them to find the hidden scientific library of the museum. There they are encouraged to look for a treasure chest. inside which they will find the final answers to satisfy their pleading senders (Fig. 5).

Game Mechanics

The game unfolds encompassing a series of game mechanics which are described below in details.

• Call for action. Upon launching the app (Fig. 1), two fictional characters -Meara and Isabel -ask the user for help (Fig. 1). Two different museum tours and game endings are delivered, depending on which character the players chooses to help. Meara leads the player to explore the marine species collected the museum, while Isabel steers them to get to know the terrestrial fauna. One character will lead the audience to discover a ghost, who has been haunting the museum for almost a century, while the other will haunt the players herself with a malignant turn of events. • Game of shadows. For the player to uncover the truth about the haunting of the museum he/she needs to interact with the species in display on the glass shelves. The players engage in a game of shadows (Fig. 2), where they have to match museum exhibits with the silhouettes proposed by the application, one at the time. By identifying the correct species from its shadow, the players gain some scientific knowledge about the animal and an Augmented Reality (AR) three-dimensional (3D) model of it, which they can manipulate and examine in detail as if it was outside the protected shelves (Fig. 4, left side). In order to identify the correct species for each shadow the game offers a tip that are available clicking on a help button, on the top right corner of the device. As the players answer quizzes and learn about the marine and terrestrial fauna of the archipelago, the audience is rewarded with pieces of a map for each carried out interaction. Once completed the map will finally disclose where they can find the answer to the mystery they have been engaged to solve. • Markers and Augmented Reality Artifacts. Not all the species in display are part of the game. Twelve special markers (six per different characters journey) have been designed in the shape of Runes (ancient Nordic divining alphabet) to contradistinguish the species that are part of the game. When the user finds the animal that he/she recons corresponding to the shadow displayed on the app, he/she can capture the Rune shaped marker (Fig. 2). If the exhibit is the correct one, the user is rewarded with several scientific facts and curiosities about that animal, and a 3D model of it.

• The AR 3D model artifact -freeing species from the shelves. Once the 3D model is triggered, the audience can manipulate it through an AR interface and explore it in details from all its sides by rotating it in all directions, zooming in into its features, complementing the knowledge gained from the observing the exemplar in the glass shelves with the curiosity generated by manipulating the AR 3D model. • Quiz. After capturing the marker and the corresponding AR 3D model, the app asks the audience to answer a quiz related to the taxidermied animal, exhibited in the shelves. The quiz questions are designed to prompt the viewer to look closely at the exhibited species as the quiz is related to some physical details of the animal. After answering the question, either rightly or wrongly, the audience is presented with a text reporting on several additional scientific facts about the animal. • The puzzle and the treasure hunt mechanism. For every completed interaction, from finding the silhouette corresponding animal, to receiving the scientific information, the audience is rewarded with a piece of a puzzle, representing a map. Once completed, the map will guide the players to a hidden location, the scientific library of the museum. In the library, the audience is encouraged to look for a small chest, containing the answer to their search for the truth. The chest is a small wooden box hidden among the scientific publications of the library. Opening the chest, reveals a last Rune, which will lead them to two different ending, depending which character they decided to help in the beginning. • Conclusion: exposing the haunting. When the user finds the treasure box and captures the last marker, the game is completed. If they had been helping Meara, they will uncover the presence of a restless ghost, once landlord of the place, haunting the museum premises (Fig. 5). If, on the other hand, the players were helping Isabel, they will be met by the evil ghost of Isabel herself (Fig. 5). She will deny them any satisfaction, take all of their points, and warn them not to wander about the museum site as it once was her house, and she doesn't like strangers walking around!. To quench her wrath and recover their points, the players are given one last option: they need to rightly answer one last question regarding the museum tour. • The reward, a token for later reflection. In the end, independently from who they have been helping, all the players receive a reward for finishing their quest. They can take a selfie photograph with the encountered ghost and their game punctuation which will be emailed to them. Moreover, they will receive a book (in PDF printable format) containing pictures and scientific facts of the animals that they have interacted with during the tour (Fig. 4). The book also contains the Runa/markers which can be recaptured at any time and release the AR 3D models of the species.

Implementation

Haunted Encounters makes use of the Unity3D engine, chosen for its flexibility and extensive community support. Some widely known applications that are built upon the Unity3D engine are Pokémon GO and Magic Arena. Unity3D was also chosen for being compatible with the Vuforia Engine, which provides a set of tools such as the use of custom images presented in the real world as in-game triggers, using the custom images' features for recognition.

The Vuforia Engine allows the recognition of these custom images by uploading them to a database and performing a feature extraction process, determining the quality of the image as a marker for Augmented Reality during the process. This results in a black and white image with emphasis on the highest contrast points. As a rule of thumb, an image with a greater number of features provides for a better target for the AR application. In terms of logic, the bulk of it is in the quiz part, where it follows the behaviour of the Factory software design pattern and builds a list of quizzes. While loading, new instances of the quiz pre-set are created and populated with the correct information relative to the displayed species. Each quiz consists of a set of questions, related to the species to which the marker is associated. To proceed, the player must select the desired species and find the correct image in the physical environment. Vuforia triggers the image recognition event, causing the application to display a question along with a set of possible answers, with a correct answer being worth full score and a wrong one only a fraction of it. Once all the questions have been answered, a new scene will load, tasking the player to find the last image. This will trigger the ending part of the application, starting the playback of videos while showing the surroundings to the player. The video format chosen was webm, for the support of transparency and its light-weight characteristics.

Evaluation

Pilot Testing and preliminary findings

Before the formal evaluation the game was piloted with two young girls of the age of 12. The experimenters followed the users during the trial taking notes and collecting loose feedback. An open-ended interview was made to the players to better understand their perception and experience with the game. With this pilot, we aimed at testing the flow of the game, general usability of the interface and gather some preliminary impressions. A summary of insights from the open-ended interview with the young teens is summarized below. Both users found the game interesting and engaging ("because we played and understood more about the animals.", C1 and ". It was fun to find the animals and take photos" C2). They found the main game challenge (matching the shadow to the correct exhibit and answering quizzes) demanding and referenced the tips as a necessary help ("Not so easy. … the tips helped." C2). C1 mentioned that she'd like to have more tips and take more photos that could then feature in the game ("we should be able to take a photo when we found the animals and put in the game… More hints instead of only shadow…"C1). One of the girls suggested to add a time challenge to the task of finding the animals corresponding to the shadows ("A time challenge that give us finite time to find the animal corresponding to the shadow" C1. ). They also appreciated the AR 3D realizations of the animals, and the fact that they could manipulate them ("They were cool" C1 and C2…" Specially because you could touch the screen and manipulate them (turn them around)".C2). The treasure hunt was their top favorite element of the game ("Looking around the museum for the animals, the treasure hunt, looking for the chest, and then the ghost thing." C2. They positively remembered the ghostly encounter at the end of the game and the game payoff, in the form of the pdf printable book ("We found a ghost it was cool." C1 and C2. it was cool because we had the book… and could capture the markers again and see the 3D animals.C1). Finally, they mentioned the game enhanced the museum experience ("In this museum, we learned much more than in other museums", C2, "I was in many museum and this idea is very cool", C1) and would recommend to friends and classmates ("Yes, but our class is quite troubling and I don't think they would behave." C1 and 2).

Preliminary Insights from the Pilot. The pilot highlighted the positive impact of the game on the young players. They were both highly engaged and had fun, mostly appreciating of the treasure hunt mechanism and learning outcomes. Attention should be paid to balance difficulties of the challenges proposed in the game with the tips given to help solve them. Overall, the flow of the game (the timing, engagement and level of challenge) worked well, but the pilot exposed an interesting addition: a time constrain to some of the challenges. The AR element were also appreciated together with the possibility of re capturing them through the PDF book markers later on. After the pilot study, we proceeded to conduct a more focused evaluation of some of the game fea-tures, before deploying an on site study of the game. In the next section, we will describe the usability evaluation carried out after the pilot, while a complete study of the game onsite is being conducted as we speak.

Formative Usability evaluation

The game was further evaluated with 16 students from a single school (14 male and 2 female -mean age: 17.25; standard deviation: 1.29). After trying the game the students compiled two different questionnaires: the Multimedia Guide Scale, measuring the reactions to the usefulness and usability of multimedia guides [START_REF] Othman | Engaging Visitors in Museums with Technology: Scales for the Measurement of Visitor and Multimedia Guide Experience[END_REF], and the AttrakDiff [START_REF] Hassenzahl | AttrakDiff: Ein Fragebogen zur Messung wahrgenommener hedonischer und pragmatischer Qualität[END_REF] measuring the attractiveness of the experience. Due to the constrained protocols and schedule of the school, the tests were set up in our lab premises where we replicated the control conditions found at the museum. This enabled us to gather data on the general usability, learnability, quality of the interaction and attractiveness of the application.

Procedure. The users were split into two even groups (8 users per group) who performed the evaluation with the same protocol into two separates but equally sized and equipped rooms. In order to interact with the mobile app., we provided one smartphone shared by every two students. In preparation for the study we photographed and printed A4 sized posters of all the museum exhibits that are covered in the Haunted Encounters game (Fig. 5). These posters contained the picture of the exhibit photographed on the museum shelf and its associated marker on the side. The lab evaluation consisted in the students playing the game in pairs, as they were in the museum, finding the requested exhibit poster and capturing its corresponding marker in order to unlock the content and answer the quiz related to each exhibit. The posters were distributed on a table in the lab room. After the interaction with the mobile app, participants were required to compile two questionnaires one for the Multimedia Guide Scale and the other for the At-trakDiff. Usability Results. Within the Museum Guide Scale (MGS) scale, the parameter of General Usability scored an average of 2.57 (median: 2.50) out of 5 points. As lower than average values in this parameter indicate participants not finding usability issues.

Hence, results show that some usability issues might be at play in the Haunted Encounters Museum game and its usability could be improved. The parameter of Learnability and Control with the mobile guide scored an average of 3.58 (median: 3.67) out of 5 points. Higher values in this parameter mean participants understood and could control the multimedia guide. Results from our test indicate that users found the application quite easy and intuitive to grasp, and master.

The parameter of Quality of Interaction with the mobile guide scored an average of 3.90 (median: 3.67) out of 5 points. Higher values in this parameter mean participants enjoyed the quality of interaction of the multimedia museum guide. In our test, the Quality of interaction scored relatively high compared to the other measures. In sum, the participants reported enjoying the experience of interacting with the application, also thanks to the intuitiveness of its feature, which helped them overcome the usability issues encountered.

The overall results from the Attrakdiff scale are very positive: this scale scores from -3 to 3. Hence, values near 3 are strongly positive. In particular, the Pragmatic quality parameters, describing traditional usability and task related to the design aspects, i.e. efficiency, effectiveness and learnability, scored 2.10 (mean 2.14). The Hedonic quality parameters (identity and stimulation), describing aspects of the product connected to the product qualities, such as originality and beauty for example, scored respectively 2.46 (mean: 2.50) and 2.21 (mean: 2.14). The Attractiveness parameter also scored highly positive with an average of 2.58 (mean: 2.64). The perceived Attractiveness of a product results from an averaging of the perceived pragmatic and hedonic qualities.

With further analysis, a significant relationship between Attractiveness and Hedonic qualities (p<0.003) emerged from the data. Attractiveness was significantly positively related to how well people scored in the Hedonic quality -identity (rs=0.748, p=0.001) and stimulation (rs=0.564, p=0.023). We can infer that the high attractiveness of our product is strongly related to its hedonic qualities (originality and beauty). On the other hand, there was not any significant correlation between the results from Pragmatic quality and Attractiveness (rs=0.398, p=0.127).

Discussion

In summary, our pilot and usability evaluations gave us an initial encouraging set of highlights and findings to guide us shape further our game and its future evaluations. In general, the game was well perceived, it was confirmed that it adds value to the museum, and encourage teenagers exploring its content. It was interesting to note how the aesthetics qualities and originality of the game, helped users overcome usability issues, and left players with an enthusiastic impression of the experience. These positive results led us to agree with [START_REF] Jemmali | Educational Game Design: An Empirical Study of the Effects of Narrative[END_REF], who argued that games are proven to improve engagement, motivation and learning within edutainment environments. Moreover, our preliminary findings echo broader literature in the field of HCI and gamified experiences for teenagers. Our findings echo [START_REF] Cesário | Shifting from the Children to the Teens' Usability: Adapting a Gamified Experience of a Museum Tour[END_REF] arguing that it is possible to have a single gamified experience for teens spanning from 12 to17 years old, when the game contains elements that are considered novel by this teens group, such as mobile applications, gamification, 3D models and interaction through AR. Teens value the technology, and appreciate when it is included in museum experiences. However, we are aware of the limitations of our study: the pilot was conducted with a very limited sample, and the usability study constrained by the reproduction of the experience in the lab, instead of the museum itself. For understanding how this mobile intervention could enhance the user experience of teenage visitors in museums, a deeper analysis is needed. The conducted studies encouraged us to continue the evaluation of the game, in particular exploring the effects of the game tested on site, and with a wider and more gender balanced sample.

Conclusions

This article reports on the design and preliminary testing of Haunted Encounters, a teenage targeted mobile game deployed at the Natural History Museum of Funchal. In the game, the audience is called to solve a mystery by looking for and interacting with the museum exhibits, collecting AR 3D models of the species in display while gaining scientific knowledge about such species. The article reports on some preliminary testing of the game and its findings. The game was piloted with two young teenagers, yielding enthusiastic reception and some suggestions on improvements. The game was then tested with further 16 digital native teenagers evaluating its usability and interaction qualities. The results were extremely positive, despite the limitation of the study which was conducted in the lab, instead of the museum premises. The next phase of our work involves extensively testing this prototype inside the museum's premises in order to validate its flow, game mechanics and overall usability on site. The results from these studies will help us understanding and facilitating the design of engaging interactive museum experiences for the understudied museum teenage audiences.

Fig. 1 .

 1 Fig. 1. The opening screen of the Memories of Carvalhal's Palace game (right side) and a screenshot of the two main characters that call the player for action (left side)

Fig. 2 .

 2 Fig. 2. One of the 12 Rune marker (right side of the figure) used to unlock the 3D model of the Butterfly. Each Rune is related to a different exhibit. This specific rune is related to the Butterfly, as it signifies "change". Screenshot of the game of shadows interface (left side of the figure).

Fig. 3 .

 3 Fig. 3. Screenshot of the ending of the game according to the two different characters that open the game: Isabel, (left) and Meara (right).

Fig. 4 .

 4 Fig. 4. Left side of the image: user interacting with the 3D model of the Butterfly displayed as Augmented Reality. Right side of the image: two pages of the digital book. It is possible to reload the 3D models of the animals by capturing the rune charms of the book pages.

Fig. 5 .

 5 Fig. 5. Teenagers using the mobile app. to interact with the poster and capture the markers, during the usability test conducted int the lab.

ACKNOWLEDGMENTS

ARDITI (Agência Regional para o Desenvolvimento da Investigação, Tecnologia e Inovação) with the PhD scholarship number M14-20-09-5369-FSE-000001. The support of the Beanstalk project team, In particular Research Assitants Rui Trinidade, Sandra Olim and Ana Bettencourt.


Suaza Juan
email: juan.suaza@correounivalle.edu.co

Trujillo María
email: maria.trujillo@correounivalle.edu.co

A health point-based dynamic difficulty adjustment strategy for video games

Keywords: Video games, Dynamic Difficulty Adjustment, Heuristics

Commonly, video games make a clear division on the difficulty offered (e.g. Easy, Medium, Hard), causing that players often do not experience what game designers intended. In this document, we propose a dynamic difficulty adjustment strategy for health point-based video games based on a heuristic evaluation performed at play time. This means that a video game may change its difficulty based on players' performance while they play. To test our strategy, we implement a fighting video game and perform an evaluation with 10 th grade students from Institución Educativa Multipropósito. Based on our results, the students considered that the game is balanced, suggesting that this strategy is a viable choice to implement and test in other video games.

Introduction

Game balance is key to provide a fun and engaged experienced to players [START_REF] Csikszentmihalyi | Beyond boredom and anxiety[END_REF]. It is defined as a state where the game is fair for a player, providing him/her with challenges that match their skill. Commonly, video games make a clear division on the difficulty offered,e.g. Easy, Medium, Hard, being these difficulties subjective and unclear, specially to new players. Therefore, players do not often experience what game designers intended. On the other hand, there are video games that have a single difficulty, to deliver the intended experience designed. In some cases, causing players to quit because it is too hard.

In this document, we propose a dynamic difficulty adjustment strategy composed of two components: a playing agent with multiple difficulties and a heuristic function that swaps between them by measuring players' performance. To test our strategy, we implement a fighting video game and perform a Player Experience (PX) evaluation. The results show that the students considered the game is balanced and each component of the strategy serves its purpose.

Methodology

Our Dynamic Difficulty Adjustment (DDA) mechanism is based on the work done by [START_REF] Brown | Developing game-playing agents that adapt to user strategies: A case study[END_REF] and [START_REF] Silva | Dynamic difficulty adjustment through an adaptive ai[END_REF], and extended to support more video games. It is composed of two core components: a heuristic function and a playing agent with multiple difficulty levels that will be swapped based on the heuristic value.

Playing Agent

The playing agent is the character players face during their play-through. This agent has difficulty levels that are interchanged based on a heuristic. A playing agent should have at least three difficulties: easy, intended, and hard.

The most common types of agents are Finite State Machines (FSMs) which allow an easy difficulty adjustment since FSMs rely on parameter values that can be changed to increase or decrease the difficulty.

Heuristic Function

A heuristic function is in charge of selecting the difficulty of the agents a player will face. To address this task, we identify two different scenarios: an immediate evaluation and a episodic evaluation:

The immediate heuristic function measures the aggression of a player using a proportion of the character's Health Point (HP) difference. Eq. 1 shows the proposed heuristic's equation.

H(P HP c , EHP c , P HP m , EHP m ) = P HP c P HP m - EHP c EHP m (1)
H ∈ [-1, 1] and it is calculated every T time-steps. P HP c , P HP m , EHP c , EHP m are the current and maximum HP levels for both characters. H = 1 means the player has won and vice versa. The goal is to keep H ≈ 0, meaning that both characters have similar HP such that they reach the end of a fight in similar conditions. For each difficulty level, different selection intervals need to be established to switch between them. For example, for three playing agents, the intervals would be: [-1, -α), [-α, α], and (α, 1].

The episodic heuristic function measures how well a player has performed in the past levels, or gameplay episodes, of a video game. This heuristic's equation is shown in Eq. 2.

H(P ) = Count(Successf ul episodes) -Count(P layer s deaths) (2)
H is calculated after an episode has finished, H ∈ Z. The count of successful episodes and player's deaths are natural numbers, including zero. If H > 0, means a harder difficulty should be selected and viceversa. An episode can be interpreted as a level, task, enemies defeated, among others, depending on the game.

The Forest of the Guardians

The Forest of the Guardians is a 2.5D fighting mobile video game developed to test the DDA proposed. The game consists on a 1 Vs. 1 fight against an agent. The agent uses a FSMs with three difficulty levels the immediate heuristic.

Finite State Machine

The enemy uses a FSM that has four core combat states, described below:

-The base attack state casts k long-range attacks in the direction of the player.

-In the move towards the player state, the agent moves towards the player.

-The melee attack state is triggered once the agent is close enough to the player's character. -Finally, in the move away state, the agent moves to the farthest corner/wall from the player's character. While in this state, if the player's character is close enough, the agent will transition to the melee attack state.

Difficulties

The three levels of difficulty proposed are: easy, medium, and hard difficulty. All of the FSMs variations have the same parameters. The parameters are shown in Table 1. These values were chosen experimentally. 4 Strategy Evaluation

Participants

Nineteen 10 th grade students from Institución Educativa Multipropósito were involved in the evaluation process (58% are males and 42% are females). All the students were asked to play the game once. The students' age ranged from 15 to 18 years-old (Mean = 16.5, SD = 1.3).

Methods

Player Experience Inventory is a standardised questionnaire proposed by [START_REF] Vanden Abeele | Design and preliminary validation of the player experience inventory[END_REF]. Player Experience Inventory (PXI) incorporates two sub-scales: the functional level, which evaluates the dynamics situated at the usage level, i.e., immediate consequences experienced; and at the psycho-social level, i.e., which exceed the usage level and focuses on the consequences at the psycho-social level.

Results and Discussion

Immersion presents positive results: 73.7% of students were not aware of their surroundings, 79% were immersed in the game, and all of the students were focused on the game. Even though there was another student playing another video game, students were focused on the fight; indicating that the Flow channel was achieved in some of them.

Challenge This dimension presents positive results, 63.1% of the students considered that the game was not too easy nor too hard to play; the remaining students show a neutral agreement. Similarly, 73.8% agree that the was challenging but not too challenging; the remaining students have a neutral agreement. Besides from the questions, a 74% win-rate is reported and an average remaining HP of 26%. Even though the remaining HP is low, which is one goal of the heuristic, the high win-rate shows that the game was probably too easy.

Considering these results we can argue that the game is considered balanced by a group of students. This means that the difficulty level assigned by the heuristic is adequate.

Conclusions and Future Work

As a result of this work, a dynamic difficulty adjustment strategy for HP-based video is proposed, including two heuristics that address different scenarios. One of the heuristics proposed was implemented in a video game and tested by 10 th grade students. Our results show that students considered the video game balanced, therefore it might provide a solution to motivate students to play a video game. In general, this strategy may be a viable solution to implement in other scenarios and video games. However, to fully establish if the strategy motivates students, longer evaluations with students are needed to corroborate that fact.

Based on our results, the immediate heuristic served its purpose: get both player to low health at the end of the fight, averaging a 26% HP remaining at the end of the fight. However, we are aware that the players' win rate is still too high (74%) implying that the FSM used was probably too easy for players.

As future work, longer evaluations are needed to validate if the proposed balancing strategy motivates students to play the game in the long-term. Moreover, the episodic heuristic should also be tested. Additionally, combining both heuristic functions is still a task to do.

Table 1 :

 1

				Difficulties' parameter values
	Difficulty	Attack	speed	(at-	Movement	speed	Armour points
		tacks/second)		(units/action)
	Easy	0.8			4		0
	Medium	1			6		2
	Hard	1.5			6		5