Prevalence is a measure of disease frequency which counts the total number of existing cases of disease within a population during a specified period of time. As it is a proportion, prevalence is often expressed in term of a percentage within a population.
- Prevalence rates for specific diseases are calculated from periodic health examination surveys that government agencies conduct. Annual changes in prevalence as reported in this booklet only reflect changes in the population; rates do not change until there’s a new survey. Changes in rates can only be evaluated with data from new surveys. 
Types of Prevalence
- Point prevalence measures the proportion of a population that is diseased at a single point in time. It is viewed as a "snapshot" of disease frequency associated to a specific time period.
- Period prevalence measures the proportion of a population that is diseased during a specified duration of time. Period prevalence differs from point prevalence in that it counts the number of cases that were present at the start of the defined time period and accounts for the number that develop over time. It is viewed more as a serious of snapshots of disease frequency associated to a string of time versus a single time point.
- Lifetime prevalence (LTP) is the number of individuals in a statistical population that at some point in their life (up to the time of assessment) have experienced a "case" (e.g., a disorder), compared to the total number of individuals (i.e. it is expressed as a ratio or percentage). Often, a 12-month prevalence (or some other type of "period prevalence") is used in conjunction with lifetime prevalence. There is also point prevalence, the prevalence of disorder at a more specific (a month or less) point in time. There is also a related figure lifetime morbid risk - the theoretical prevalance at any point in life for anyone, regardless of time of assessment. (example: Synopsis of article on "How Prevalent Is Schizophrenia?" from Public Library of Science)
- Prevalence utilizes two key assumptions:
- The numerator (number of existing cases of disease) accounts for all currently living cases 'regardless of disease onset.
- The denominator (number in total population) accounts for everyone in the population studied. This includes sick, healthy, at-risk and non-at-risk.
- Prevalence, as a value, is a proportional measurement and can only between zero to one or 0-100%.
Relationship to Incidence
- P/(1-P) = IR * D (where: P is prevalence, IR is incidence rate, and D is duration of time)
- Therefore, P = IR * D