Background: Maintenance of high participation rates in longitudinal studies is critical to their validity because of the possibility of bias associated with non-participation, which may differ between studies. This paper examines factors associated with participation status over time, by comparing the characteristics of non-attenders and attenders at a 20 year follow-up examination in a cardiovascular cohort study with an initial participation rate of 78 percent.
Methods: A baseline examination was carried out between 1978 and 1980 and subjects have been followed up through regular reviews of general practice records, postal questionnaires, and a clinical re-examination of survivors in 1998-2000. Data obtained by questionnaire in 1996 (Q96) and at baseline examination have been used to compare the characteristics of these recent non-attenders and attenders who remain in the cohort for further follow-up.
Results: The non-attendance rate of available survivors at re-examination was 23 per cent (n = 1313). Rates of non-attendance were highest in Scotland and lowest in Southern England. Non-attenders were older than attenders, more likely to smoke, and reported more disabling conditions and greater use of multiple medications at Q96, but recalled similar rates of diagnosed coronary heart disease. Marked differences in social factors were identified. Total mortality rates within 1 year of re-examination were over three times higher amongst non-attenders than attenders.
Conclusions: Non-attendance is related to health status, risk factor status and social circumstances and may affect some estimates of disease prevalence, but does not appearto have a major impact on the estimated prevalence of coronary heart disease. Information collected on non-attenders in longitudinal studies helps to estimate, adjust and minimize these effects.