This article identifies and discusses criteria that can be used by health professionals to critically appraise research articles that estimate the prevalence or incidence of a disease or health problem. These guidelines will help determine the validity and usefulness of such community assessment studies. The criteria relate to the validity of the study methods (design, sampling frame, sample size, outcome measures, measurement and response rate), interpretation of the results and applicability of the findings. The research question "What is the prevalence of dementia in Canada?" is used as an example for this paper.