2. Understanding Epidemiologic Concepts

2.1. Definitions

  • Health event: A particular disease, injury, other health condition or attribute.

  • Population: The total number of persons living in a particular place (e.g. a town or country), or the total number of persons in a particular group (e.g. with the same job or educational background).

  • Prevalence: The total number of cases of a health event in a specified population.

  • Prevalence rate: The proportion of a population that has a health event:
    •  at a specified point in time (e.g. on a certain date) – ‘point prevalence’, or
    • during a specified period (e.g. over 12 months) – ‘period prevalence’.

  • Proportion: A ratio between health events occurring and population, often shown as a fraction or percentage. For example, the number of people who have a disease compared with the total number of people studied.

  • Incidence: The number of new cases of a health event during a given period in a specified population.

  • Incidence rate: The frequency with which new health events occur, related to a particular time frame (e.g. the number of new cases per year). Incidence rate is worked out by dividing the number of new cases over a specified period either by:
    • the average population (usually ‘mid period’ - the population half-way through the period being looked at), or
    • ‘person-time’ - a measure of the number of persons at risk and the time they were at risk.
For example, in a study, the average population would be the number of participants in the study at the half way point. Person-time is the number of participants in the study, accounting for the amount of time that they were actually in the study.