Almost all of the cohort members (99.6%) can be linked to the unique patient number in the NIVEL Primary Care Database after verifying a match on sex and birthdate. Even though the invitations were addressed personally, it turned out that 113 cohort members were not the originally invited participants, but most likely another adult from the same household who did want to participate sellckchem instead of the originally invited person. Since these 113 participants were eligible (ie, 31–65 years old, living in the Netherlands), they are treated as regular cohort members, of whom 58 can also be linked to the NIVEL Primary Care Database. We observed that most of the invitees responded on the

day they received the letter or the day after that and that the (timing of the) reminder was effective (see figure 2), which is something to consider when planning the capacity of an online questionnaire and of personnel responding to questions.

Figure 1 Flow chart of recruitment and participation. Figure 2 Timing of response (online registrations) in days after receiving the (A) invitation or (B) reminder. Baseline results Table 1 shows the baseline characteristics of the cohort members. We set out to recruit participants across the Netherlands to enhance contrast in environmental and occupational exposures, urbanisation level and socioeconomic factors. The mean and median age at baseline was 51 years (SD 9.4 years). Compared with the source population, the cohort members consist of more females (56%) and older subjects (50 plus years). We observed that the participation rate varied between the general practices (the 10th and 90th centiles were 9% and 23%, respectively) and also varied per level of urbanisation of the general

practice location, varying on average from about 11% to 19% in the most and the least urban areas, respectively. This is also reflected in the distribution of cohort members by the level of urbanisation (less urban, more rural) compared with reference data from Statistics Netherlands (table 1). Nevertheless, we did succeed in recruiting participants across the Netherlands and with the varying level of urbanisation, as depicted in figure 3. Table 1 Baseline characteristics of the AMIGO cohort members (N=6561 men, and N=8268 women) Figure 3 Geographical spread of the Occupational and GSK-3 Environmental Health Cohort Study (AMIGO) across the Netherlands at baseline. Legend: number of AMIGO cohort members (dots) and level of urbanisation per municipality. 1=Very high (on average >2500 addresses … The majority was employed, never smoked cigarettes and did drink alcoholic beverages in the past 12 months (table 1). The fast majority was born in the Netherlands (97%). Those with intermediate levels of completed education are somewhat under-represented among cohort members compared with reference rates from Statistics Netherlands for 35–65-year-olds in 2012 (table 1).