Spacing of follow-up waves in incidence studies.

Longitudinal studies of dementia are often undertaken to estimate the incidence of dementia and to identify incident cases for the study of risk factors measured on the entire cohort at baseline. The power of these analyses is determined primarily by the number of demented cases identified. Increasing the number of waves of evaluation of the cohort in a given time period increases the yield of demented cases but it also increases the cost of the study. This paper provides a method for estimating dementia incidence in the presence of mortality and loss to follow-up. It also assesses the trade-off between the cost (as measured by the number of re-evaluations) and the yield of incident dementia cases so that longitudinal studies of dementia can be designed optimally in the future. Copyright 2000 John Wiley & Sons, Ltd.