Figure 1.

Absolute difference in log OR comparing models using all controls with models using controls responding before first and second reminders, against risk factor prevalence. Each point represents the absolute difference in the log OR for a potential risk factor compared with an analysis using all available controls. Closed circles: models using controls returning a questionnaire before being sent a reminder; Open squares: models using controls returning a questionnaire before being sent a second reminder. All models are additionally adjusted for sex, age group, study site and month.