Definition of terms
| Outcome | Health related event of interest—for example, death, having an STI, or having AIDS |
| Exposed | Having, or being exposed to, a potential cause or risk factor for disease or other outcome—for example, being a smoker or having sex with someone who has an STI |
| Unexposed | Not having, or being exposed to, a potential cause or risk factor for disease or other outcome |
| Prevalence | A measure of the burden of disease in a population, calculated as the number of individuals with a disease or outcome divided by the total number of individuals in a defined population at a given time. Prevalence is expressed as a proportion |
| Incidence | A measure of the rate at which new cases of a disease occur in a population, calculated as the number of new cases divided by the population at risk over time. Incidence is expressed as a rate per unit time (for example, annual rate) or as a number per person years at risk |
| Odds | The ratio of the probability of an event occurring to the probability of the event not occurring |
| Odds ratio | The ratio of two odds. The odds ratio calculated from a case-control study is the ratio of the odds of exposure among the cases to the odds of exposure among the controls |