Prevalence

Prevalence

While incidence measures the frequency of a new event in a population, prevalence provides a measure of the frequency of existing cases of a disease or other condition in a population.¹ Measures of incidence tend to be more useful for analyzing causal associations, but prevalence measures are useful for determining the burden of disease and planning for the provision of health services.⁴

The point prevalence may be calculated as P/N, where P is the number of individuals with a disease at a given time and N is the number of individuals in the population.⁴ This type of information might be collected through a survey conducted at a particular point in time.¹ The period prevalence is a measure of the frequency of a disease or condition in a population over a given time period.¹ When calculating the period prevalence, the numerator includes existing cases at the start of the time period and new cases which arise during the time period, while the denominator is the average reference population over the period.¹

Prevalence measures may be used to describe the frequency of behaviour, patient characteristics, or health services use.⁴ For example, an analysis of the prevalence of antipsychotic use among children age 14 or younger in British Columbia showed that the one-year period prevalence of antipsychotic use increased from under 0.05% of children in 1997 to over 0.35% of children in 2007.¹¹