The occurrence of a given disease in a population is measured by prevalence and incidence. Prevalence reflects the burden of a given disease in a specific population and is calculated as point prevalence or period prevalence. Incidence denotes the number of new cases of a given disease which occur in a cohort of individuals followed up for a given period of time, and may be expressed in terms of cumulative risk or incidence rate. The relationship between disease frequency (e.g., myocardial infarction) and specific exposure (e.g., smoking) is assessed by calculating relative (relative risk and odds ratio) and absolute (risk difference) measures of effect.