Calculate period prevalence from unique cases and the population represented, or solve for cases or population using percent and per-population reporting scales.
Period Prevalence Formula
Period prevalence measures the proportion of a population that had a condition at any time during a defined period.
Period Prevalence (\%) = (Number of Cases) / (Population at Risk) * 100
Number of Cases = (Period Prevalence) / (100) * Population at Risk
Population at Risk = (Number of Cases) / (Period Prevalence / 100)
- Period Prevalence (%) = percentage of the at-risk population with the condition during the period
- Number of Cases = people who had the condition at any point during the period
- Population at Risk = people who could have developed or had the condition during the period
The period must be clearly defined, such as one month, one year, or a study interval. The population at risk should match that same period and setting.
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 at-risk population had the condition during that year.
Do not count the same person more than once for the same condition in the same period.
