84 | Chapter 1 3 male (0). Period and age are measured as time-varying, continuous variables indicating the calendar year and the respondent’s age, ranging from 1948 to 2010 and from 18 to 35 years, respectively. Period thus tells us whether the risk of stopping a sport increases or decreases over the course of time. Educational level is available in three categories: low (1), middle (2) and high (3). We included it in the analyses as a continuous variable, to keep our models parsimonious. Migration background consists of three categories, based on the definition of Statistics Netherlands (Alders, 2001): native Dutch origin (1) and non-native (those with at least one parent who was born abroad) of non-Western (2) and Western (3) origin. A numerical description of all variables is found in Table 3.1.