Chapter 3. Do birds of a feather play football together? 59 Club membership An individual is considered a member of an amateur football club when he or she is officially registered at the Royal Dutch Football Association as a club member during a playing season. The length of the playing season was defined as beginning on the 15th of August of a certain year and ending on the 15th of May in the next year. Memberships which commenced after the 15th of May but were terminated before the 15th of August were left out. While rare, in some cases individuals have multiple club memberships. Given the focus on club compositions, these additional memberships are included in the study. Please note that the total number of memberships reported in the results therefore slightly exceed the number of individuals connected to these memberships. Furthermore, to avoid including clubs which are inactive and/or only exist on paper, I used a threshold of a minimum of 30 registered members in a given playing season. Segregation Ethnic inbreeding in co-membership ties can be measured using segregation indices, as these are primarily designed to measure the extent to which populations are unequally distributed over lower-level units. For the purpose of this study, I use the index information theory index H, developed by Henri Theil (Theil, 1972; Theil & Finizza, 1971). In a review of six multigroup segregation indices, Reardon & Firebaugh (2002) conclude that the information theory index is the only measure following the “principle of transfers”, which means that transfers of members to clubs with a lower proportion of ingroup members would be reflected in a decline of the index. The information theory index is an entropy-based measure, meaning its calculation is based on entropy score E, sometimes described as the diversity index or score (Hao & Fong, 2011; Iceland, 2004). The entropy score expresses the degree of uncertainty about group membership when randomly selecting an individual from a population, assuming mutually exclusive groups. This degree of uncertainty is both a function of groups’ proportions in a population and the total number of groups, and can be expressed in the following way (Theil, 1972): = ∑ (− ) =1