Thierry Baudasse

Rémi Bazillier

Thierry Baudassé

« Migration and Trade Union Rights »

Keywords: F22, J51, J61, J83 Migration, Core Labor Standards, Freedom of Association and collective bargaining

We study in this paper both theoretically and empirically the influence of trade union rights in origin countries on bilateral migration flows. Theoretically, we propose two complementary models. In the first model, trade union rights are supposed to increase the bargaining power of workers. We model these rights as a transfer from high-skilled workers to low-skilled workers, assuming that this latter category of workers will benefit more from freedom of association and collective bargaining. However, we do have to take into account the large extent of informal economy in lots of developing countries. If trade union rights are only enforced in the formal sector, workers from this sector will benefit from a wage premium. The most qualified will then be the first winners of an improvement of such rights if they are more employed in the formal sector. We then propose different alternative indexes measuring trade union rights. We find that, all things being equal, more trade union rights tend to be associated with less migration of low-skill and high-skilled workers. Effects are not significant for intermediate skill level. Lastly, we show that social tensions may have the opposite effect. If trade union rights are associated with more social instability, it may increase the level of migration. It emphasizes the importance of social dialogue.

Résumé

Nous étudions dans ce papier l'influence des droits syndicaux dans les pays d'origine sur les flux de migration bilatéraux, tant du point de vue théorique qu'empirique. Théoriquement, nous proposons deux modèles complémentaires. Dans le premier modèle, les libertés syndicales sont supposées améliorer le pouvoir de négociation des travailleurs. Nous modélisons cet effet comme un transfert des travailleurs qualifiés vers les travailleurs qualifiés, considérant que cette catégorie de travailleurs bénéficiera plus de la liberté d'association. Néanmoins, nous devons également prendre en compte la large place de l'économie informelle dans les pays en développement. Si les libertés syndicales sont seulement respectées dans le secteur formel, les salariés de ce secteur bénéficieront d'un salaire plus élevé. Si les qualifiés sont relativement plus employés dans ce secteur, ils bénéficieront plus d'une amélioration de ces droits. Nous proposons ensuite différents indicateurs mesurant la liberté syndicale. Nous trouvons que, toutes choses égales par ailleurs, une meilleure liberté syndicale est associée avec moins de migration des qualifiés et des nonqualifiés. Les effets ne sont pas significatifs pour une éducation intermédiaire. Enfin, nous montrons que les tensions sociales peuvent avoir les effets opposés. Si les droits syndicaux sont associés à une plus grande instabilité sociale, ils peuvent augmenter le niveau de migration. Cela met en avant l'importance du dialogue social dans la mise en place de la liberté syndicale.

I.

Introduction

In 2006, 4 millions of permanent migrants entered OECD countries (OECD, 2008). It represents an increase of 5% compared to the previous year. In average, immigrants counted for 12% of the whole population in OECD countries. Since 2000, the increase is very high in some countries: 66% in Ireland, 40% in Finland and 34% in Austria. Political debates on these issues are very sensitive and relatively new in some countries which were emigration countries and became immigration countries in the last years. On the other side, emigration represents a huge challenge for developing countries. In some countries, for example in Caribbean islands, more than 40% of the working force lives abroad [START_REF] Defoort | Tendance de long terme des migrations internationales, analyse a partir des 6 principaux pays receveurs[END_REF]). The problem is more accurate for educated workers with emigration rates over 70% (85% in Jamaica). Challenge is huge both for destination and origin countries. In immigration countries, the integration of migrants into local labor markets is a real concern. In source countries, governments may be concerned by brain drain and its consequences on general human capital accumulation.

Thus the understanding of migration determinants is a central point for decision makers. In particular, wages and labor market characteristics are of particular interest.

Migration may be influenced by economic or social characteristics in source and destination countries. We may distinguish push factors from pull factors. The conventional wisdom considers that migrants are attracted by generous social welfare, high wages or good working conditions. However, labor markets characteristics in source countries do not have to be neglected. This paper addresses the influence of labor standards in origin countries on emigration rates.

More precisely, out of the four core labor standards3 recognized by ILO (1998), freedom of association and collective bargaining rights potentially have one of the largest effects. We propose to see how working conditions in origin countries may influence the decision to migrate.

The economic literature on migration deals with two main questions. The first one is the determinants of labor migration. This tradition comes from [START_REF] Hicks | The Theory of Wages[END_REF] who argued that the main determinant of migration is the difference of wages. [START_REF] Todaro | A model of labor migration and urban unemployment in less developed countries[END_REF] and [START_REF] Harris | Migration, unemployment and development: a two sector analysis[END_REF] observed that even in the presence of urban unemployment, migration is still an attractive option if the agents maximize their expected earnings. Field (1975) and [START_REF] Gupta | Rural-Urban migration, informal sector, and development policies[END_REF] introduced an informal sector into this framework. The "new economics of migration" proposes new possible determinants of migration. For example, movement of workers can be a risk diversification strategy inside the household. According to this approach, income of migrants, which is not correlated with agricultural income, will reduce the risk for the household [START_REF] Levhari | On migration and risk in Less Developed Countries[END_REF]Lucas and Stark, 1985;[START_REF] Stark | The Migration of Labour[END_REF]. Another useful hypothesis introduced by new economics of migration is that individuals are not migrating only in response of absolute income differential but also according to relative deprivations [START_REF] Stark | Migration as a response to relative deprivation[END_REF][START_REF] Stark | Relative Deprivation and Migration[END_REF][START_REF] Stark | The Migration of Labour[END_REF].

The second main question is the consequences of migration. One major issue is the influence of workers migration on the labor market in destination countries (see for example [START_REF] Grossman | The substitutability of natives and immigrants in production[END_REF][START_REF] Borjas | The Economics of Immigration[END_REF][START_REF] Borjas | The Economic Analysis of Immigration[END_REF][START_REF] Borjas | How much do immigration and trade affect labor market outcomes[END_REF]. Another important issue is the influence of migration on the accumulation of human capital in source countries. This concern began with [START_REF] Bhagwati | The Brain Drain and Income Taxation[END_REF] and [START_REF] Bhagwati | The Brain Drain, International Integration of Markets for Professionals and Unemployment: A Theoretical Analysis[END_REF], who argued that migration may have negative consequences in developing countries because of the deprivation of their best-trained workers (phenomenon of brain drain). Nevertheless, more recent models [START_REF] Beine | Brain Drain and economic growth: theory and evidence[END_REF][START_REF] Beine | Brain Drain and LDC's Growth: Winners and Losers" The Institute for the Study of Labor[END_REF][START_REF] Mountford | Can a Brain Drain be Good for Growth in The Source Economy?[END_REF][START_REF] Stark | A Brain Gain with a Brain Drain[END_REF] show brain drain may have positive consequences, through increased incentives to accumulate human capital in developing countries. Brain drain may become a brain gain. Finally, some authors have questioned the effects of migration on social capital in destination countries as well as in source countries [START_REF] Schiff | Trade, Migration and Welfare: The impact of social capital[END_REF]2002).

Interactions between labor markets have been studied in many different ways but focused on two main specific aspects: the role of wages differential and the consequences of migration on labor markets in destination countries. So far, very few studies focus on the influence of labor market characteristics in source countries. In parallel, a controversial debate emerged concerning the development outcomes of labor standards. The empirical literature on this topic established the ambiguous links between labor standards and international trade [START_REF] Brown | International Trade and Core Labour Standards: A Survey of the Recent Literature[END_REF][START_REF] Granger | Normes de travail fondamentales et échanges Sud-Nord[END_REF], foreign direct investment [START_REF] Kucera | Core Labour Standards and Foreign Direct Investment[END_REF], economic coordination [START_REF] Aidt | Core Labour Standards and Economic Development : impact on longterm per-capita income[END_REF], productivity [START_REF] Brown | International Labour Standards and Trade : a theoretical analysis[END_REF][START_REF] Maskus | Should Core Labor Standards be imposed thought international trade policy[END_REF], OCDE 1996), long-term per capita income [START_REF] Bazillier | Core Labour standards and inequalities, is there a social Kuznets curve[END_REF]) and income inequalities [START_REF] Bazillier | Core Labour standards and inequalities, is there a social Kuznets curve[END_REF]. Most of these outcomes may influence the determinants of emigration. Lastly, the extent of informal economy matters regarding the economic consequences of trade union rights and migration. [START_REF] Calvo | Urban Employment and Wage determination in LDC's : Trade Unions in the Harris-Todaro Model[END_REF] introduced trade unions in the [START_REF] Harris | Migration, unemployment and development: a two sector analysis[END_REF] framework where the rural-urban migration depends on the wage-setting influenced by trade unions. [START_REF] Quibria | Migration, Trade Unions, and the Informal Sector: A Note on Calvo[END_REF] introduced explicitly the informal urban sector in such framework. [START_REF] Harrison | Labor Markets in Developing Countries : An Agenda for Research[END_REF] showed that restrictive labor standards may induce a shift of a significant part of the working force from the formal to the informal sector. [START_REF] Singh | The global labour standards controversy : critical issues for developing countries[END_REF] considered that freedom of association may increase the level of informality through the fall of formal employment and labor demand in this sector. Empirically, the influence of labor standards on informality is not clear. Marques and Pages (1998) found a high correlation while [START_REF] Kucera | Labor Standards and Informal Employment in Latin America[END_REF] showed that countries with higher standards in Latin America tend also to have a higher share of formal employment.

This paper proposes to study the influence of freedom of association and collective bargaining rights4 on migration in source countries. The first contribution of this paper is the modeling of the possible effects of collective bargaining on emigration rates. Collective bargaining is alternatively modeled as (1) an income transfer from the high-skilled workers to the low-skilled and, (2) a premium for workers in the formal sector. The second contribution of the paper is the building of an original measurement of freedom of association and collective bargaining right, for a large panel of countries. The third contribution is the empirical analysis of such linkages. Globally, we find a negative impact of freedom of association on emigration rates for high-skilled workers and low-skilled workers.

The paper is organized as follow. The second section proposes different theoretical modeling of the effects of collective bargaining on migration flows. The third section deals with the empirical measurement of freedom of association and collective bargaining rights.

Fourth section deals with the econometric analysis of their effects on migration. A fifth section proposes a complementary analysis dealing with the influence of social tensions on migration. Finally, last section concludes.

II. Theoretical frameworks

A. Presentation of complementary models

We may suppose that collective bargaining will induce an improvement of wages and working conditions for workers at the bottom of the pay scale 5 . However, if this right is only respected in the formal sector, workers with the lowest productivity may not benefit from these collective rights if they are excluded from this sector6 . We will thus propose two complementary modeling of these effects. First, collective bargaining rights will be modeled as redistribution from high-skilled to low skilled workers. In order to raise wages of low skilled workers, firms will reduce the relative wages of the high-skilled workers in order to maintain their level of profit (otherwise, the capital will fly outside the country). A second option could be the following one: the improvement of working conditions in the formal sector will increase the size of the informal sector where these rights are not respected [START_REF] Harrison | Labor Markets in Developing Countries : An Agenda for Research[END_REF]. High-skilled workers will move to the formal sector which will employ workers with the highest productivity. Low-skilled workers will not benefit from this right and the winners will be high-skilled workers. We will then have two possible models: a model where collective bargaining is modeled as a transfer from high-skilled to low-skilled workers (model a) and a model where collective bargaining is a premium for workers in the formal sector, with no change in the informal sector (model b). Empirical analysis will be used to validate or refute each of them.

B. Collective bargaining as a transfer from high-skilled to low skilled workers

We consider two countries S and N (respectively the South and the North). In each country, population is standardized to 1. There exist in these two countries two categories of workers: skilled (index q) and unskilled (index nq). The proportion of skilled workers in the north is α and in the South is β, it is supposed that α> β. In the North the production function is:

Y = A 1 . L q + A 2 . L nq with A 1 > A 2 (1)
And in the South it is :

Y = B 1 . L q + B 2 . L nq with B 1 > B 2 (2)
We suppose that labor standards are perfectly respected in the North7 and/or we are under perfect competition. Wages, w, are established as follow:

w q = A 1 (3) w nq = A 2 (4)
In the South, labor market is characterized by imperfect competition. Employers have a market power over unskilled workers. This market power is equivalent to redistribution from unskilled to skilled workers. We assume that the lack of freedom of association creates a situation of monopsony on the labor market (OECD 1996(OECD , 2000)). The enforcement of labor standards will then compensate this distortion on the labor market.

Skilled workers are in proportion β, so a dollar taken from an unskilled worker ends in (1-β)/β additional dollar for each skilled worker.

There are two different wages in the south:

v q = B 1 + (1-β)/β . t 1 (5) v nq = B 2 -t 1 (6)
t 1 is the transfer from unskilled to skilled workers. It reflects the level of freedom of association enforced in the country. The closer to zero is the value of t 1, the higher is the enforcement of such right. So, diminishing the value of t 1 gives us what we formerly call formally « model a ».

Individuals in the South are characterized by a certain level of migration cost, depending on their educational level: c i (k) with k= (q, nq). This cost is supposed to depend on the psychological capacity to support distance with friends and family, the existence of migrants' network abroad, the individual taste for mobility. Parameter c i in each category of workers is distributed according to a uniform distribution between 0 and one maximum value C*(k) with k= (q, nq).

Individuals decide to migrate when:

w k ≥ v k + c i , that is when w k -v k ≥ c i .
They migrate when the cost of migration is lower than wage differential between north and south. For a given value of the parameters, there will exist a proportion of the population of the category equal to (w k -v k )] / C* (k) such that this proportion of the population is willing to migrate.

c i Z w k -v k C* 1 / C*
By decreasing t 1 one decreases v q and thus one increases w q -v q and (w q -v q )] / C* (q). In other words one increases the proportion of skilled workers who wish to migrate. On the other hand, one increases v nq and thus one decrease the proportion of unskilled who want to migrate.

Proposition 1: if collective bargaining rights are respected in all sectors and if wages only

depend on skill levels, the enforcement of such rights will increase the number of skilled migrants and will lower the number of unskilled migrants.

The proportion of each population wishing to migrate is then defined by the following table:

Qualified Not-qualified F / C*(q) with F= A 1 -B 1 -(1-β)/β . t 1 G / C* (nq) with G = A 2 -B 2 + t 1
Notice that an alternative model would be the following. In absence of collective bargaining, unskilled workers are paid below their productivity (v nq = B 2 -t 1 ) while skilled workers are paid at the level of their marginal productivity (without wage premium as stated before).

Here, the transfer t 1 would be a rent for owners of firms. In this alternative model, collective bargaining leads to a decrease in the number of unskilled migrants inasmuch as their income is higher than in absence of trade union rights. However, number of skilled migrants would remain unchanged.

Proposition 1bis: In a model similar to the one of Proposition 1, when there is no wage premium for the skilled in absence of collective bargaining rights, the enforcement of such rights will lower the number of unskilled migrants but will remain the number of skilled migrants unchanged.

B. Collective bargaining as a premium for the formal sector (model b)

We now have two sectors: the informal and the formal sectors. Each worker is characterized by a level of qualification q i . These levels q i are distributed according to a uniform distribution between 0 and a maximum value Q. Formal sector attracts skilled workers because of better wages and working conditions. Firms from the formal sector can choose the most productive workers and then hire the most qualified. Other workers work in the informal sector. In order to simplify, we assume that wages in the formal sector are equal to the sum of a minimum institutional real wage w and a premium which depends on the qualification of the employee: w(q i ) = w+p(q i ), with p(0)=0 and p'(q i )>0. The result of collective bargaining is a rise of the minimum wage w. We also assume that the formal sector produces an internationally tradable good which price is determined by the international price. Income in the informal sector is supposed to be equal to s<w. We assume that informal sector produces a service. The economy is then composed by two goods: the tradable good produced by the formal sector and the service produced by the informal one.

When w raises all wages in formal sector raise and consequently the marginal cost of the good raises for a given produced quantity. Maximization of profit implies that the price of the good (internationally fixed) is equal to the marginal cost so, in order to maximize profit, the formal sector will shrink its production and employment. So, we have:

L f = f(w) with f'<0, L f

being the employment in formal sector

A rise in w will then imply a fall in formal employment L f . The formal sector will then select more strictly its employees, hiring the individuals with a level of qualification higher than q i *, in order to have:

L f / L = (Q -q i *)/ Q, with L the total employment, equal to the total population which is constant. By hypothesis, there is no unemployment, the informal sector being able to hire the population not employed in formal sector with a wage s.

q i L f / L q i * 1 / Q Q
Obviously, when L f decreases, the minimum level of qualification q i * needed to be hired in the formal sector rises. The increase of the minimum wage w, which is here a consequence of collective bargaining, induces a higher selectivity in the formal sector. In summary, in this framework, only skilled workers will benefit from this labor standard. It can possibly have negative consequences for unskilled workers if they are fired from the formal sector.

Collective bargaining will then increase the income of all skilled workers and it may decrease the income of some unskilled. This will lead to less migration among skilled workers and it may lead to more migration among the unskilled.

Proposition 2: if collective bargaining rights are only enforced in the formal sector and if the wages depend crucially on the sector of employment, the enforcement of such rights will lower the migration of skilled workers and possibly increase the migration of the unskilled.

III. Measurement of trade union rights

Different measures exist in order to estimate a level of freedom of association. These indexes can be classified into two main categories: legal or outcome indexes.

Legal indexes observe legal protection of trade union rights. One limit of this family of indexes is that it gives no information about the effectiveness of these rights. Even if freedom of association is a constitutional right, there is no guarantee that this right is enforced in practice. On the other side, a country without legal protection may face large activities of trade unions or collective bargaining. In this category of indexes, we have [START_REF] Botero | The Regulation of Labor[END_REF] indexes on labor union power and collective bargaining.

Outcome indexes are based on the consequences of freedom of association on the number or proportion of workers that belong to an organization related to labor issues [START_REF] Ghai | Decent Work : concepts and indicators[END_REF]. "In general, the higher the union density is, the stronger the defense of workers interests in negotiations with employers and the government, and the greater the participation by workers in matters affecting their work". However, there are different problems.

Unionization rate is also based on historical traditions or political systems [START_REF] Jose | Organized Labour in the 21st century[END_REF]. Ghai ( 2003) also argues it is a problem for developing countries because of the small size of the labor force in the formal economy. Moreover, a high unionization rate does not reflect necessarily a good level of freedom of association if the membership to a trade union is compulsory, strongly recommended, or needed to get access to some jobs. Activities of trade unions can be strong where unionization rate is low (like in France). We will then take into consideration other outcome variables such as the number of strikes or proportion of workers involved. Lastly, the number of strikes reflects both trade unions activities but also the level of social tensions in the country.

Here A criterion frequently chosen to decide how many factors to retain is the Kaiser criteria. According to this criterion, if a factor explains more than the original variable, it is necessary to extract it. As the sum of eigenvalues of p variables is equal to p, we only retain factors which have an eigenvalue higher than 1. According to this criterion, only the first factor would be sufficient to get a satisfactory description of the data. However, we could notice that the second and the third factor has also a significant explicative power and an eigenvalue very close to one. This result reinforces the idea that trade union rights are a very complex phenomenon, difficult to measure and to describe. Our explanation is that one single information (for example a high proportion of workers involved) may reflect two things. First, it may mean that trade union rights are well-respected. But it can also mean that the level of social dialogue is weak and workers do have to use a power struggle to obtain by demonstrating what they would have obtained through negotiations. It may also mention social tensions in the country. This ambivalent nature of trade union rights outcome variables cannot be avoided. The task to obtain a good proxy of trade union rights is thus difficult. We will take into account this difficulty in our empirical strategy.

However, the observation of the variables coordinates on the different axis may help us to distinguish these different aspects (Table 2). As we can see, the coordinates on the first factor may be interpreted as a proxy of trade union rights and the intensity of trade unions activities. The number of strikes, of workers involved influences positively the coordinates on this factor. The civil rights index has the opposite influence (a high value of this index reflects a weak enforcement of these rights) while the index of [START_REF] Botero | The Regulation of Labor[END_REF] has the opposite effects (more collective bargaining rights induce a positive coordinates on the first axis). We will then use the coordinates on this axis as a first proxy of trade union rights.

It is difficult to give a meaning to the second factor because of contradictory information. If workers involved in trade unions activities have a negative influence on its coordinate on this axis, the number of strikes has the opposite effect. We have the same contradiction with the two other indexes which have both positive coordinates while the meaning is the opposite. We will thus do not exploit this index.

However, the third axis gives interesting intuitions. Here, workers involved, strikes and low civil rights have a positive influence on the coordinates while the level of collective bargaining rights has a negative influence. We will thus interpret this axis as a proxy of "social tensions" for positive coordinates on this axis and "social dialogue" for negative coordinates. Countries may have very low number of strikes and workers involved if social dialogue is strong and trade union rights respected. At the contrary, numerous strikes or workers involved may be interpreted as a signal of social tensions. As we can see in table 1, the contribution of this axis is very closed to the one of the second axis (0.23 against 0.24).

We call index_TU_4 this index.

By interpreting factor 1 and 3, we use more than 55% of the information included in the variables. [START_REF] Botero | The Regulation of Labor[END_REF] Because of the difficulty to measure effectively the level of trade union rights, we propose to use in the empirical sections different alternative indexes. Two of them are constructed with the same methodology as index_TU. The three others are proposed by other authors [START_REF] Botero | The Regulation of Labor[END_REF][START_REF] Kucera | Measuring Trade Union Rights: A country-level Indicator Constructed from Coding Textual Sources[END_REF], CEPII 2001).

The index index_TU_2 aggregates the unionization rate, the number of strikes per year, the number of workers involved in trade unions activities (in % of the working force), and the index civil rights, from Freedom House. We do not include index_col_barg13 in order to have only variables measuring the outcome of trade union rights and not legal indexes. As stated before, the main limit of this index is the limited number of countries for which we can compute this index because of limited statistics concerning unionization rate. Table 3 presents the results of PCA. As for index_TU, the first axis would reflect the general level of trade union rights while the second axis will be interpreted as a proxy of social tensions in the country.

We then propose to build a third index without the unionization rate (to include more countries) and without index_col_barg13 (in order to have an outcome index). Table 4 gives the result for this index index_TU_3. And lastly, we will use the index proposed by [START_REF] Kucera | Measuring Trade Union Rights: A country-level Indicator Constructed from Coding Textual Sources[END_REF]

IV. Empirical analysis: trade union rights and migration

A. Empirical specification and data

We want to measure the influence of freedom of association and collective bargaining on bilateral migration flows, by skill-level. We then propose to estimate the following model:

(7)
where is the migration rate (in percentage of the working force of the country i) in log, from country i to country j, for workers with skill level e (respectively primary, secondary or tertiary). is the level of collective bargaining in country i. is a vector of control variables specific to the country i. is a vector of control variables specific to the country j. is a vector of bilateral control variables. is the error term. We assume it to be i.i.d.. Standard errors are clustered at the origin-destination level and robust to heteroscedasticity.

All estimations firstly use OLS estimators and standard errors are clustered at the origindestination-couple-level 10 . However, we should take into account one important feature of our migration database which is the high proportion of zero for the dependant variable (26% in the total population database) that may lead to inconsistent estimates. The use of a log 9 0 if no rights guaranteeing freedom of association-if rights exist, from 1=weak rights, little freedom to 4=strong rights, substantial freedom. 10 We do not present however the results of OLS estimate. Results are relatively similar to the one using Heckman two-steps procedures. Moreover, the inverse Mills ratio is significant in most estimations, putting in evidence biased OLS estimates. Tables of results are available upon request.

specification will drop all zero observations creating biaised estimates. This problem has often be ignored in the literature on migration while it is relatively common in the international trade literature [START_REF] Linders | Estimation of the Gravity Equation in the Presence of Zero Flows[END_REF]. Some recent papers on migration deals with the high proportion of zero and propose econometric strategies to correct this biais [START_REF] Beine | Diasporas[END_REF][START_REF] El Yaman | Migrations et investissements directs étrangers dans l'espace européen[END_REF]. We thus propose to use [START_REF] Heckman | Sample Selection Bias as a Specification error[END_REF] Control variables include the level of GDP per capita in origin and destination country.

This variable can be interpreted in two ways. It may be a proxy of wages. It is also a proxy of migration costs. If income is too low, workers do not have the capacity to migrate. We also add the proportion of young people within the population considering this is the more mobile group (with less migratory costs). We take into account the level of democracy, measured by a combined polity score (Polity IV) proposed by [START_REF] Gleditsch | Modified Polity P4 and P4D Data[END_REF] 12 to capture migration costs linked to the political system. We also control for the population in origin and destination countries. We add bilateral variables such as the existence of common frontiers, distance between countries, the fact to have a common language, and the fact to have a past colonial past. All these variables are correlated with the existence of a network of migrants and then will influence the migratory costs. Finally, we also add regional dummies in order to capture 11 As noticed by [START_REF] Wooldridge | Econometric Analysis of Cross-Section and Panel Data[END_REF], the problem to not use an additional instrument is the high correlation between the Mills ratio and the other variables in the second equation. This will lead to a lower significance of the coefficients. 12 See [START_REF] Gleditsch | A re-examination of democracy and autocracy in modern politics[END_REF] for a detailed presentation of the index. Basically, this is a combined index of several sub-dimension measuring different aspects of 'authority' (competitiveness of political participation, regulation of political participation, competitiveness of executive recruitment, openness of executive recruitment, constraints on chief executive). It is included between -10 (autocracy) and 10 (democracy).

partially unobserved heterogeneity between countries.

B. Results

Table 5 gives the result of the estimations using our first index of trade union rights (index_TU). We observe a significant and negative relation between trade union rights and emigration rates. This effect on the overall migration flows is explained by a significant impact on the emigration rate of primary educated workers and tertiary educated workers. Our two models presented in the theoretical section of this paper are then validated conjointly by our estimates. Low-skilled workers may benefit from trade union rights through an increased bargaining power, higher wages and better working conditions. On the other side, high-skilled workers may also benefit from trade union rights because of a wage premium in the formal sector. Effects are not significant for secondary educated workers for whom one effect may be compensated by the other.

Other control variables globally take the expected sign. The GDP per capita in origin countries has a negative influence on emigration rate, except for low-skilled workers where this variable is not significant. This can be explained by the existence of too high migration costs for this category of workers. Population in origin countries is negatively correlated with emigration rates while population in destination countries has a positive influence. Political factors seem to have a significant role: autocracy increases migration costs. However, this effect is not significant for skilled workers. Concerning bilateral variables, only the fact to have a former colonial relationship has a significant (and positive) impact on bilateral migration flows. However, we may suppose that the effect of other variables (such as the common language) is also captured by this variable.

This first result is important. For different reasons, trade union rights may retain both lowskilled and high-skilled workers. We should however be cautious at this stage in the interpretation of our results because of the difficulty to measure effectively trade union rights.

That is why we propose to test, in a second time their robustness by using alternative indexes of trade union rights, presented in the previous section.

C. Robustness checks of the results: alternative indexes of trade union rights

We only present the estimated coefficient of different trade union rights indexes. Estimated coefficients of control variables are globally similar with the results presented in table 5.

Both legal indexes and outcome indexes confirm the previous result: trade union rights has a negative impact on bilateral migration flows both for primary educated workers and tertiary educated workers. The only difference is the result obtained with the index of trade union rights violations proposed by [START_REF] Kucera | Measuring Trade Union Rights: A country-level Indicator Constructed from Coding Textual Sources[END_REF]. The result is inverted. Less violations of these rights would increase the migration rate, both for primary and tertiary educated workers. We will see in the next section that this result can be explained by the heterogeneous consequences of trade union rights.

V. Migration, social tensions and social dialogue

As we saw in section III, outcome variables may reflect two things:

(1) the level of trade union rights which we measure through our index index_TU, but also (2) the level of social tensions which is captured by the third factor of the PCA (index_TU_4). As we can see in figure 1, different profiles of countries can be detected.

Countries such as Argentina or Bolivia for example are characterized by a high value of their coordinates on the first and second axis. This can be interpreted by a high level of social tensions. For relatively similar coordinates on the first axis, countries such as Sweden, Norway or Japan have a negative coordinates on the second axis. This is a characteristic of countries where level of social dialogue is high and thus, the number of strikes is limited.

Until now, we only focused on trade union rights. But we can suppose that social tensions may have different effects. Violations of trade union rights measured by [START_REF] Kucera | Measuring Trade Union Rights: A country-level Indicator Constructed from Coding Textual Sources[END_REF] may also be linked to this level of social tensions. Violations of these rights are of course negatively correlated with their respect. But it may also be positively correlated with the general level of social tensions in the country. We then propose to measure the influence of this aspect in the migration decision, by estimating conjointly the effect of trade union rights as such through index_TU, and the effect of social tensions measured by index_TU_4 and Kucera index. India India India India India India India India India India India India India India India India India India India India India India India India India India India India India India Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Finland Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Japan Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden

PCA analysis of trade unions rights

Freedom of association -social dialogue -social tensions Note: The PCA analysis includes as variables: the number of strikes per year, the number of workers involved in trade unions activities (in % of the working force), the index civil rights (Freedom House), and the index of [START_REF] Botero | The Regulation of Labor[END_REF]. The results confirm this intuition. If trade union rights still have a negative impact on migration (index_TU), our index index_TU_4 which is a proxy of social tensions has the opposite effect. We find the same result when we use the Kucera index of trade union rights violations. The only difference is the positive sign is significant for primary educated workers when we use the Kucera index while it is significant for tertiary-educated workers when we use Index_TU_4. The sign and the significance of both coefficients are similar when these indexes are integrated conjointly in the specification.

Trade union rights can be an effective tool to retain workers in their country. It is interesting to notice that this result is also confirmed for high-skilled workers. trade union rights may be a tool for countries fighting against brain drain. However, the existence of trade union rights is not a sufficient condition for retaining workers. If the development of trade union rights came together with a development of social tensions, it can have the opposite effects.

Instability is a factor of emigration. Political instability may be a factor of emigration. It is also true for social instability.

VI. Conclusions

In this paper, we show theoretically two channels of transmission from trade union rights to migration. The first one is an increased bargaining power for workers and more especially low-skilled workers. The second one is the existence of a wage premium in the formal sector that will benefit mainly to high-skilled workers. Empirically, we propose different measures of trade union rights. We find that these rights in origin countries have a negative impact on emigration rates, all things being equal, for low-skilled workers and high-skilled workers.

Effects are not significant for secondary-educated workers. Both theoretical models are then validated by our estimates. However, we also put in evidence that trade union rights may have heterogeneous consequences. The development of such rights may increase the level of social dialogue but it may also increase the level of organized strikes and demonstrations. As it is the case for political instability, social disorders may be a factor of emigration. The second effect may counterbalance the first one.

If the development of trade union rights may be seen as a political instrument for retaining workers in their home country, and particularly for avoiding brain drain, the government should also propose a framework favorable to the development of social dialogue and nonconfrontational collective bargaining mechanisms.

  two-step regressions providing consistent estimates in the case of selection bias. The first step is a probit estimate of the probability to have a positive migration flow (selection equation). Estimations using Heckman two steps strategy generally propose an additional instrument for this selection equation. However, as stressed by[START_REF] Wooldridge | Econometric Analysis of Cross-Section and Panel Data[END_REF], the use of an additional instrument in the selection equation is not strictly necessary. As it is very difficult to find a convincing instrument which may explain the decision to migrate but not influence the size of migration flows, we decide to run two-steps Heckman estimates without additional instrument 11 . Data on migration are from[START_REF] Docquier | A gender assessment of the brain drain", dataset 1990-2000 with gender breakdown[END_REF] and are available for 1991 and 2001. Due to the small number of period, we do not include time or country fixed effects.

Figure 1 :

 1 Figure 1: PCA analysis of trade union rights and social tensions

Table 1 : PCA results for index_TU

 1 which measures the statutory protection and power of union 8 .In a first time, we do not propose to include the unionization rate into this index because of the limited number of countries where this information is available. We will however present in a second time different alternative indexes to check the robustness of our results. The is obtained using principal component analysis (PCA), which goal is to isolate common factors between different variables (here the effective trade union rights), by reducing total information in order to get an easier economic description of the variables. Table1gives the results of this PCA.

	Component Eigenvalue Difference	Proportion Cumulative
	Factor 1	1.31421	0.329148	0.3286	0.3286
	Factor 2	0.985065	0.0481961	0.2463	0.5748
	Factor 3	0.936869	0.173017	0.2342	0.8090
	Factor 4	0.763853	.	0.1910	1

, we choose to aggregate different categories of indexes measuring both legal protections and effective enforcement of trade union rights. Our first index index_TU aggregates the number of strikes per year, the number of workers involved in trade unions activities (in % of the working force), the index civil rights, from Freedom House measuring among other things the effective level of trade union rights, and an index proposed by

[START_REF] Botero | The Regulation of Labor[END_REF]

, index_col_barg13, 8 It measures the statutory protection and power of unions as the average of the following seven dummy variables which equal one: (1) if employees have the right to unionize; (2) if employees have the right to collective bargaining; (3) if employees have the legal duty to bargain with unions; (4) if collective contracts are extended to third parties by law; (5) if the law allows closed shops; (6) if workers, or unions, or both have a right to appoint members to the Boards of Directors; and (7) if workers' councils are mandated by law. index

Table 2 : Coordinates of variables on main factors

 2

	Variable	Factor 1	Factor 2	Factor 3	Factor 4
	workers involved	0.5676	-0.1553	0.5561	0.5869
	Civil Rights Freedom House	-0.4619	0.7205	0.1502	0.4950
	Strikes	0.4975	0.6384	0.2374	-0.5371
	index_col_barg13	0.4658	0.2217	-0.7822	0.3493

Table 3 : PCA results for index_TU_2

 3

	Component Eigenvalue Difference	Proportion Cumulative
	Factor 1	1.76356	.622041	0.4409	0.4409
	Factor 2	1.14151	.491461	0.2854	0.7263
	Factor 3	.650054	.205177	0.1625	0.8888
	Factor 4	.444876	.	0.1112	1
	Variable	Factor 1	Factor 2	Factor 3
	TU_rate	0.5406	-0.3376	0.6636
	workers involved	0.6162	0.2767	0.0719
	Civil Rights Freedom House	-0.5327	0.3108	0.7439
	Strikes		0.2103	0.8443	-0.0321

Table 4 : Results of PCA (index_TU_3)

 4 AFD), constructed from a world survey conducted with MINEIE and AFD agencies present in the countries covered in the database. The variable used, freedom of association, is coded from 0 to 4 9.

	Component	Eigenvalue Difference	Proportion Cumulative
	Factor 1	1.18537	.159728	0.3951	0.3951
	Factor 2	1.02564	.236653	0.3419	0.7370
	Factor 3	.788989	.	0.2630	36526
		Variable	Factor 1	Factor 2
	workers involved	0.7295	-0.0141
	Civil Rights Freedom House -0.4490	0.7443
		Strikes	0.5159	0.6677
	The fourth index is the one created by Botero et al. 2004, index_col_barg13, which
	measures the statutory protection and power of unions. This index is clearly a legal index
	measuring only the legal protection of trade unions. It is included as a variable in the
	index_TU.

The fifth index is proposed by CEPII (2001) from a database built by researchers from the French Ministry for the Economy, Industry and Employment (MINEIE) and the French Development Agency (

  . His method is based on coding violations of these rights recorded in what are regarded as the three best existing textual sources on trade union rights (the International Confederation of Free Trade Unions' (ICFTU) Annual Survey of Violations of Trade Union Rights, the United States State Department's Country Reports on Human Rights Practices, and the ILO's Report of the Committee on Freedom of

	Association).

Table 5 : Heckman two-steps estimates of bilateral migration flows

 5 The dependent variables are the logarithm of migration rates (in percentage of the source country total working force).

	Dep. Var	lnmig	select	lnmig_prim	select	lnmig_sec_	select	lnmig_ter	select
	index_TU	-0.142***	0.0425	-0.211***	0.0309	-0.0504	0.0466	-0.216***	0.0635*
		(-3.417)	(1.090)	(-4.653)	(0.821)	(-1.209)	(1.229)	(-5.427)	(1.647)
	lngdp_o	-0.122*	0.104*	0.0903	0.107*	-0.214***	0.101*	-0.400***	0.121**
		(-1.852)	(1.802)	(1.233)	(1.908)	(-3.227)	(1.797)	(-6.272)	(2.123)
	lngdp_d	2.659***	1.403***	2.404***	1.581***	2.220***	1.458***	3.013***	1.448***
		(10.61)	(18.45)	(7.428)	(20.76)	(7.921)	(19.42)	(11.38)	(19.11)
	lnpop_o	-0.432***	0.224***	-0.421***	0.245***	-0.496***	0.247***	-0.379***	0.223***
		(-9.857)	(7.727)	(-7.991)	(8.691)	(-10.20)	(8.707)	(-8.672)	(7.851)
	lnpop_d	1.099***	-0.0475**	1.023***	0.0512**	0.938***	0.00134	1.114***	-0.00912
		(46.78)	(-2.054)	(36.83)	(2.371)	(39.74)	(0.0616)	(50.22)	(-0.411)
	lnyoung	0.00211	-0.108***	-0.0236	-0.110***	-0.168***	-0.0851***	-0.0499	-0.102***
		(0.0629)	(-4.553)	(-0.623)	(-4.759)	(-5.168)	(-3.672)	(-1.546)	(-4.358)
	lnedu	0.550***	0.0281	0.568***	0.0742	-0.388***	0.0987	0.479***	0.0452
		(5.156)	(0.321)	(4.731)	(0.872)	(-3.583)	(1.153)	(4.665)	(0.525)
	polity	0.0472***	-0.000222	0.0654***	6.85e-05	0.0394***	0.000698	0.00550	-0.00288
		(4.103)	(-0.0216)	(5.121)	(0.00693)	(3.421)	(0.0696)	(0.503)	(-0.285)
	colony	0.509***	0.150	0.610***	0.138	0.476***	0.101	0.237	0.141
		(3.284)	(1.014)	(3.585)	(0.964)	(3.078)	(0.713)	(1.613)	(0.972)
	contig	0.160	-0.0463	-0.119	-0.00732	0.176	0.0483	0.0160	-0.0210
		(0.558)	(-0.179)	(-0.384)	(-0.0284)	(0.621)	(0.188)	(0.0594)	(-0.0814)
	-0.0695 comlang_ethno	-0.0379	0.0410	0.00384	-0.138	-0.0417	0.119	-0.0185
		(-0.552)	(-0.376)	(0.294)	(0.0389)	(-1.087)	(-0.424)	(0.992)	(-0.185)
	dist	-4.91e-06	-5.50e-06	-1.89e-05	-1.05e-05	1.51e-06	-5.14e-06	4.73e-06	-6.20e-06
		(-0.462)	(-0.591)	(-1.608)	(-1.174)	(0.142)	(-0.572)	(0.468)	(-0.679)
	america	0.265**	-0.0977	0.287**	-0.0415	0.241*	-0.0646	-0.000968 -0.0850
		(2.034)	(-0.821)	(1.997)	(-0.361)	(1.848)	(-0.558)	(-0.00780) (-0.729)
	africa	0.470***	-0.213	0.494***	-0.238*	0.398**	-0.126	0.975***	-0.207
		(2.820)	(-1.520)	(2.623)	(-1.752)	(2.409)	(-0.915)	(6.086)	(-1.500)
	pacific	0.205	0.414*	0.432	0.313	0.199	0.181	-0.0668	0.330
		(0.853)	(1.828)	(1.643)	(1.490)	(0.836)	(0.873)	(-0.295)	(1.516)
	europe	1.492***	0.0304	1.194***	0.106	1.747***	0.0530	1.608***	0.0260
		(8.986)	(0.198)	(6.508)	(0.716)	(10.51)	(0.357)	(10.19)	(0.173)
	lambda		-1.031**		-1.008*		-0.991**		0.0774
			(-2.160)		(-1.909)		(-1.999)		(0.159)
	Constant	-48.53***	-16.82***	-47.30***	-20.92***	-35.82***	-18.93***	-49.32***	-18.14***
		(-14.49)	(-14.34)	(-10.42)	(-17.79)	(-9.153)	(-16.32)	(-13.63)	(-15.56)
	Observations	2856	2856	2856	2856	2856	2856	2856	2856
	z-statistics in parentheses
	*** p<0.01, ** p<0.05, * p<0.1
	Note:

Table 6 : Heckman two steps estimates of bilateral migration flows

 6

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	United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States United States

  Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Norway Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Bolivia Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark Denmark France France France France France France France France France France France France France France France France France France France France France France France France France France France France France France Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina Argentina is the first axis of the PCA. Index_TU_4 is the third one. Labels marked are for year 2001.

	index_TU_4 (social dialogue vs social tensions)	-2 -1 0 1 2 3	-2	0	2	4
				index_TU (freedom of association)

index_TU

Table 7 : Heckman two steps estimates of bilateral migration flows -trade union rights and social tensions Dep.Var.

 7

		lnmig	select	lnmig_prim select	lnmig_sec	select	lnmig_ter	select
	index_TU	-0.146***	0.0405	-0.212***	0.0281	-0.0512	0.0435	-0.238***	0.0612
		(-3.497)	(1.027)	(-4.657)	(0.735)	(-1.226)	(1.133)	(-5.970)	(1.568)
	index_TU_4	0.0356	0.0147	0.0112	0.0195	0.0122	0.0228	0.177***	0.0184
		(0.872)	(0.387)	(0.249)	(0.528)	(0.298)	(0.617)	(4.592)	(0.492)
	Observations	2856	2856	2856	2856	2856	2856	2856	2856
	index_TU	-0.154***	0.0386	-0.244***	0.0283	-0.0641	0.0428	-0.210***	0.0558
		(-3.650)	(0.976)	(-5.275)	(0.743)	(-1.510)	(1.115)	(-5.257)	(1.424)
	kucera	0.0365*	0.00594	0.0843***	0.00481	0.0330	0.0153	0.0211	0.0144
		(1.810)	(0.323)	(3.785)	(0.270)	(1.622)	(0.857)	(1.108)	(0.805)
	Observations	2797	2797	2797	2797	2797	2797	2797	2797
	index_TU	-0.159***	0.0359	-0.247***	0.0248	-0.0657	0.0385	-0.234***	0.0521
		(-3.749)	(0.893)	(-5.303)	(0.639)	(-1.541)	(0.985)	(-5.864)	(1.310)
	index_TU_4	0.0402	0.0175	0.0232	0.0214	0.0162	0.0277	0.176***	0.0248
		(0.985)	(0.457)	(0.518)	(0.575)	(0.396)	(0.740)	(4.608)	(0.656)
	kucera	0.0380*	0.00718	0.0852***	0.00626	0.0337*	0.0173	0.0272	0.0162
		(1.879)	(0.385)	(3.816)	(0.347)	(1.654)	(0.953)	(1.433)	(0.890)
	Observations	2797	2797	2797	2797	2797	2797	2797	2797
	z-statistics in parentheses
	*** p<0.01, ** p<0.05, * p<0.1

, Université d'Orléans. thierry.baudasse@univ-orleans.fr

, Université d'Orléans. remi.bazillier@univ-orleans.fr

Freedom of association and collective bargaining, elimination of child labour, forced labour and discrimination.

Freedom of association and collective bargaining rights are considered as equivalent. The recognition of these rights is included in the same ILO conventions (87 and 98). Freedom of association, collective bargaining and trade union rights will be understood similarly. No distinctions will be made between the use of the three terminologies.

According to Saint Paul (2002), unskilled workers are often more unionized and benefit more from labor market institutions.

See the literature on "insiders" and "outsiders" theories[START_REF] Lindbeck | Insiders versus Outsiders[END_REF], for an overview).

Of course, this is a strong asumption. However, as we focus on the influence of trade union rights in the sending country, we consider it is not a problem in our case.


Nazih Ouwayed

Abdel Belaïd

François Auger

General Text Line Extraction Approach based on Locally Orientation Estimation

Keywords: Handwritten Arabic documents, text line segmentation, skew angle estimation, Snake, Wigner-Ville distribution, overlapping and touching lines

This paper presents a novel approach for the multi-oriented text line extraction from historical handwritten Arabic documents. Because of the multi-orientation of lines and their dispersion in the page, we use an image paving allowing us to progressively and locally determine the lines. The paving is initialized with a small window and then its size is corrected by extension until enough lines and connected components were found. We use the Snake for line extraction. Once the paving is established, the orientation is determined using the Wigner-Ville distribution on the histogram projection profile. This local orientation is then enlarged to limit the orientation in the neighborhood. Afterwards, the text lines are extracted locally in each zone basing on the follow-up of the baselines and the proximity of connected components. Finally, the connected components that overlap and touch in adjacent lines are separated. The morphology analysis of the terminal letters of Arabic words is here considered. The proposed approach has been experimented on 100 documents reaching an accuracy of about 98.6%.

INTRODUCTION

Ancient handwritten Arabic documents are for the most of them multi-oriented. In fact, in the meantime, the documents were required to be completed. Thus, annotations have been added. For problems of space, these additions have been accomplished in the margins creating multi-oriented text boxes with sinuous lines (see Figure 1).

In this context, classical approaches for text line segmentation cannot succeed. Already, all methods, global in nature and working at the page level, like the projection techniques, 1 the nearest neighbor clustering technique, [START_REF] Likforman-Sulem | Extracting lines on handwritten documents by perceptual grouping, in advances in handwiting and drawing: multidisciplinary approach[END_REF] Hough transform, 3 etc. must be discarded. Besides, the consideration of Arabic document needs some specific precaution. In fact, the local orientation of isolated characters or PAWs (Part of Arabic Words) does not obligatory correspond to the main orientation of the global word. Some ligatures are vertical or oblique while the word is horizontal. This is why we oriented our research towards local techniques. This paper is organized as follows: In Section 2, our approach for text lines extraction using a multiple steps (paving, oriented zone detection, text line extraction and separation of connected lines) is detailed. Experiments results are showed in Section 3 and last section concludes the paper.

SYSTEM OVERVIEW

The system operates following several steps. First, an automatic paving is operated dividing the page image in windows of the same size. Then, the orientation is estimated in each window using the energy distribution on the local projection histogram profile. Afterwards, the oriented zones (a zone is a set of neighbor windows with the same orientation) are detected. Finally, the lines are extracted in each zone using a method based on the follow-up of connected components. The connected adjacent lines are separated in a post-processing step using statistical information about terminal letters of Arabic words.

Further author information: (Send correspondence to A.A.A.) A.A.A.: E-mail: nazih.ouwayed@loria.fr, abelaid@loria.fr, Telephone: +33 3 83 59 20 61 B.B.A.: E-mail: francois.auger@univ-nantes.fr

Paving

In this step, the document image is partitioned in small windows of (w × h) size (see Figure 2.c). This size is automatically generated, based on the idea that a window must approximately contain 3 lines in each window, to be able to produce a histogram profile representative of the orientation. This follows several steps. First, an initial window of arbitrarily size (15 × 15 pixels) is placed in the middle of the image, supposed representing a horizontal alignment (see Figure 2.a). Then, the Snake approach is applied to determine the lines (see the explanation below). The window width is enlarged until the snake will give at least 3 lines. Once the lines found, the average line height h is performed as well as the average gap ḡ between them. The gap distance is calculated using the convex hull-based metric 4 (see Figure 2.b). The final window size is equal to (w × h) where w = h = 3 × h + 2 × ḡ.

Active Contour Model (Snake)

The active contour models is defined as an energy minimizing spline. [START_REF] Kass | Snakes: Active contour models[END_REF] From an initial contour, a deformation of this contour is operated to make the most closest possible of the initial shape. The energy functional minimized is a weighted combination of internal and external forces. The internal forces stems from the shape of the snake, while the external forces emanates from the image.

The snake is parametrically defined as v(s) = (x(s), y(s)) where x(s), y(s) are x, y co-ordinates along the contour and s is from [0,1]. The energy functional to be minimized is:

E * snake = 1 0 E snake (v(s))ds = 1 0 [Eint(v(s)) + Eimage(v(s)) + Econ(v(s))]ds (1)
where E int is the internal spline energy that serves to regulate its shape, E image is the external spline energy which attracts the snake to the contours sought in the image, and E con which expresses some additional constraints that may be imposed by the user to the snake he wants to get.

The traditional external energy is located on the external contours. This forces to initialize the snake near the target contour. Moreover, the gradient values are opposite from the two sides of the same contour, which prevents the snake entering the concavities. For that reason, Xu et al. [START_REF] Xu | Gradient vector flow: A new external force for snakes[END_REF] developed a new kind of external energy that permits the snake to start far from the object, and forces it into boundary concavities. This energy is named gradient vector flow, or GVF. We used the GVF on the major axis of the connected components within each line of the window (see Figure 3).

Oriented zone detection

This step consists to extract first the local orientation in the windows and to extend them carefully to the neighborhood. These two steps runs as follows.

Orientation estimation in the windows

Usually, we perform the projection profile histogram to determine the local orientation in the window. However, the presence of noise in raw document images can create local maxima which disturb the projection histogram analysis. Hence, to avoid this drawback, we propose to use Cohen's class distributions 7 on the projection histogram profiles obtained using different projection angles. These distributions reduce the importance of these local maxima and are fitted to the non-stationary nature of these histogram profiles.

We focused on the Wigner-Ville distribution (WVD), whose properties can be more responsive to the presence of peaks that the other distributions of Cohen's class (see equation 2).

W x (t, f ) = +∞ -∞ x(t + τ /2)x * (t -τ /2)e -j2πf τ dτ (2)
To estimate the orientation angle, we use the analytic signal x a (t) of the centered projection profile x(t) of the document or of the selected area. The analytic signal is the signal x(t) without its negative frequencies.

The profile x(t) is determined by projecting each document with a chosen orientation. To calculate all possible profiles, we turn the image around its center of gravity (which gives us a point deduced from the image content and not from its size and framing) and we choose the horizontal axis as an arbitrary reference for the zero degree angle. Then, we compute a time-frequency representation for each projection profile.

The angle corresponding to the profile with the highest maximum value of its time-frequency representation is chosen as the estimated angle of the document. Figure 4

Window correction and extension

Two cases may happen after the orientation calculation:

The first case occurs when the orientation is satisfactory. Hence, we try to extend the window area to its neighbors. For this, we perform the orientation of the window reassembled with each one of its neighbors. If the orientation of the total is equal to the orientation of one of them, we really merge them. In the opposite, we keep the first configuration (see figure 6.a).

The second case is related to non orientation satisfactory. In this case, we try to split the window in smaller ones allowing us to reach better precise orientation.

Text line extraction

The text line follow-up starts in the first window on the right side of the page. The algorithm starts by looking for the new maxima (see Figure 5.a). Each peak represents the starting point P s of the baseline bl j . The ending point P e of the baseline is calculated using the P s , the orientation, the width and the height of each window (see figure 5.b). The baseline bl j is calculated basing on the two points (P s , P e ) and the orientation of the window. The connected components that belong to a baseline are looked for construct the text line (see Figure 5.c).

A step of text line correction follows the text line detection to assign the non-detected components and the diacritical symbols to the appropriate text line (see Figure 5.c andd). A distance method is used to address this problem. First, the distance between the centroid of non-detected component or diacritical symbol C i and the text line is calculated. C i is assigned to the text line l j if d ci,lj < d ci,lj+1 else to l j + 1. For each zone (see figure 6.a), the text lines are clustered to form the zone text lines. Then the relations between the text lines zones are studied to form the document text lines (see figure 6.b). For this, we see if a connected component is up to two text lines between two zones. If it is the case, we merge the two text lines in one text line

Separation of connected lines

In Arabic, the lines can be connected essentially at the level of terminal letters in the PAWs (Part of Arabic Word). Hence, we propose a method considering the morphology of these letters. To face this connection problem, the analysis will be focused on the connection zones (see figure 7).

The zones are determined considering a rectangle around the intersection point S p of the two connected components which size is fixed manually. The starting ligature point B p is the highest point in the zone close to the baseline. The descender direction is determined according to B p relative to S p . According to these characteristics, our idea consists to follow the skeleton pixels within the zone using the starting point B p and the right descender direction. The follow-up will then cross the intersection point S p and continues in the right direction that we have to determine. Figure 8 shows the final result of the separation algorithm (for more details see [START_REF] Ouwayed | Separation of overlapping and touching lines within handwritten arabic documents[END_REF] ).

EXPERIMENTAL RESULTS

To study the effectiveness of our approach, we experimented it on 100 handwritten Arabic documents that contain 2500 text lines. They are manuscripts belonging to the Tunisian National Library, 9 National Library of Medicine in the USA 10 and the National Library and Archives of Egypt. 11 The tests were prepared after a manual indexing of documents by zones and text lines using a truth files. The orientation experimented varies between -75 • and +90 • . The tests were carried out on a PC equipped with a microprocessor Pentium M 1.4 GHz and 1 GB of memory under Windows XP. The application has been developed with MATLAB R2007b. In the multi-skew (zones) detection level, we have achieved a level of accuracy of 97%, this rate increases to 99% if we don't take care with the small zones that are not detected. The 1% error rate is due to the paving and the false inclination. In the text line segmentation level, the accuracy reaches 98.6%. The 0.5% of text lines not detected is due to the multi-skew detection algorithm. The 0.9% error rate is due to the presence of diacritical symbols in the beginning of the lines that create false maxima. Figure 9 illustrates the usability of our algorithm on a sample of 3 documents chosen arbitrarily from the 100 documents processed. To identify the lines, each consecutive pair of lines is presented by two different colors.

CONCLUSION

An original approach is proposed in this article, which aims to extract the text lines from the multi-oriented handwritten Arabic documents. First, the multi-skewed zones are detected using an automatic paving and the Wigner-Ville Distribution. Then, the text lines are extracted based on the orientation of each zone and the baselines. Finally, the connected adjacent lines are separated using an approach based on the morphology analysis of the terminal letters of Arabic words. The 98.6% of extraction rate show the efficacy and the performance of our approach. In the next step, we will try to generalize this approach to latin, urdu, persian scripts and to documents containing multiples scripts.

Figure 1 .

 1 Figure 1. Examples of multi-oriented ancient Arabic documents.

Figure 2 .

 2 Figure 2. Automatic paving algorithm (d1,d2,d3 are the gap distances).

Figure 3 .

 3 Figure 3. Application of the Snake for line detection, (a) Major axis drawn for each connected component of the lines. The ellipse encapsulates the initial connected component of (b), (c) shows the deformed snake of (b), (d) gives the final result showing the connected components gathered in each line.

  shows the highest values of the Wigner-Ville distribution (WVD) of the histogram profiles of the document shown on the the top-left level of the figure.

Figure 4 .

 4 Figure 4. Energy intensity of the text window in the top-left of the Figure (true angle is +15 • ).

Figure 5 .

 5 Figure 5. Text line detection steps for a window, (a) maxima detection, (b) baselines estimation, (c) assignment of each connected component and diacritical symbol to its appropriate line, (d) extracted lines.

Figure 6 .

 6 Figure 6. (a) Multi-skewed zones detection, (b) text lines extraction of document in Figure 2.a.

Figure 7 .

 7 Figure 7. Writing direction according to the terminal letters morphology (true direction indicated by red arrow and the false by blue).

Figure 8 .

 8 Figure 8. (b) Overlapping and touching connected components separation result of the window in (a).

Figure 9 .

 9 Figure 9. Some results of the multi-oriented text line extraction approach.

Table 1 .

 1 Results of the multi-skew estimation for the documents of the Figure13.

	Figure	Document	Resolution (dpi)	w × h of paving (pixels)	Execution time (s)	Zone number
		size		a	b	c	a	b	c	True	Detected
											a	b	c
	a	390×632	96	56×35	75×75	50×50	57	34	37	5	5
	b	572×800	72	52×53	75×75	50×50	77	59	84
	c	750×820	120	125×75	90×90	50×50	52	47	141
	d	362×500	72	91×36	120×120	50×50	39	26	57