Reviews of prevalence and incidence are predominantly derived from observational studies. A cross-sectional study is the appropriate study design to determine the prevalence of a particular health problem. Cross-sectional surveys are typically used to estimate the point prevalence of common conditions of long duration and are generally not appropriate for rare or temporary diseases. As incidence is the number of new cases of a particular illness within a population over time study participants need to be followed up. Therefore, cohort studies that have a prospective or longitudinal design and follow up each subject over a suitable period of time are the best way to establish the incidence of a disease or the natural history of a condition. However many study designs may provide prevalence and incidence information.