The substantial majority (82.6%) of women are known to have been invited to enrol during the 1990–92 recruitment campaign. Subsequent work to define the ‘eligible sample’ identified additional pregnancies where the offspring were eligible for recruitment, but no replies to recruitment in 1990–92 had been received (either positive or negative). With funding to complete a ‘Focus@7’ follow-up assessment of all of the participants at 7 years of age, the opportunity was taken to attempt to recruit all known eligible children who would have fitted the original eligibility criteria, excluding those who had previously refused enrolment. Invitations describing the study were sent to this group, inviting the mothers to enrol (recruitment Phase II in Figure 3). In addition to this systematic recruitment, ALSPAC made subsequent opportunistic contact with additional families who were ‘eligible’ but not ‘enrolled’ (recruitment Phase III in Figure 3). Contact was initiated by eligible families seeking enrolment or during ALSPAC community outreach and promotion activities. Due to the nature of these recruitment methods, it is not possible to determine how many individuals were ‘invited’ during recruitment Phases II
