Calculate period prevalence from the number of unique cases and the population over a period, as a percent or per 1,000 to 100,000, or solve for cases.
Period Prevalence Formula
Period prevalence measures the proportion of a population that had a condition at any time during a defined period.
- Period Prevalence (%) = percentage of the population represented who had the condition at any time during the period
- Number of Cases = unique people who had the condition at any point during the period, not episodes or repeat visits
- Population Represented = the population corresponding to the same case definition, setting, and period, including people counted as cases; it is not restricted to people initially free of the condition
The period must be clearly defined, such as one month, one year, or a study interval. Use a denominator appropriate to that study and period. This calculator uses whole-person, unweighted counts, with unique cases contained in the population represented. A changing population may require a study-specific average or mid-interval denominator; the calculator does not choose one. When adding pre-existing and new cases, the groups must not overlap. Inverse calculations can give fractional estimates when the supplied prevalence is rounded; the nearest whole-person estimate rounds halfway upward and does not create observed cases. Zero cases and zero prevalence do not identify a unique positive population, and zero cases with positive prevalence cannot determine one.
Useful Tables
| Measure | What it tells you | Example wording |
|---|---|---|
| Point prevalence | Cases present at one specific time | Prevalence on January 1 |
| Period prevalence | Cases present at any time during a period | Prevalence during 2024 |
| Incidence | New cases occurring during a period | New diagnoses in 2024 |
| Prevalence | Equivalent proportion | Approximate interpretation |
|---|---|---|
| 1% | 0.01 | 1 in 100 people |
| 5% | 0.05 | 5 in 100 people |
| 10% | 0.10 | 10 in 100 people |
| 25% | 0.25 | 1 in 4 people |
Example
If 80 people in a population of 2,000 had a condition at any time during one year, the period prevalence is:
(80 / 2,000) × 100 = 4%
That means 4% of the population represented had the condition during that year.
Do not count the same person more than once for the same condition in the same period.
