86 Ethnic sorting in football exposure may weaken the impact of ingroup share and outgroup composition on members with a minority background, because minorities are on average more accustomed to outgroup members than the other way around. This leads to my final hypothesis: H6: Migrant background mitigates the associations between ingroup share and dropout, and between outgroup fractionalization and dropout 4.3 Methodology Data This study is part of a research project for which individual membership data of all registered amateur football clubs in the Netherlands between the years 2005 and 2015 was provided by the Royal Dutch Football Association (KNVB). These membership data were combined with microdata on individual background characteristics of Dutch inhabitants from Statistics Netherlands12. The resulting longitudinal dataset spans ten playing seasons, which start on the 15th of August and end on the 15th of May in the following year13. To prevent false records of individual member dropout - for instance due to club mergers or dissolutions -, only clubs that have existed during the complete observation period are included in this study. Additionally, to ensure measurement reliability and comparability, clubs with fewer than 30 members in any of the playing seasons were excluded from the data. Because dropout of members is determined by comparing membership data to the subsequent season (see below), data of the tenth playing season was only used for construction of the dependent variable. Consequently, the final study sample contains 10,205,331 individual observations distributed over nine playing seasons and nested within 2778 clubs. Measures 12 All presented results are based on calculations by the author using non-public microdata from Statistics Netherlands. Under certain conditions, these microdata are accessible for statistical and scientific research. For further information: microdata@cbs.nl. 13 True start and end dates of playing seasons fluctuate slightly based on season, region, level and team performance. In consultation with the Royal Dutch Football Association, these dates were found to be most reasonable.
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