Prevalence
Proportion of a population that has a particular condition at a point in time.
Formula
Variables
Example
1 500 cases in 300 000 people → 500 per 100 000
How it works
Prevalence tells you how common a condition is in a population at a specific time. It is found by counting everyone who currently has the condition, dividing by the total number of people in the population, and then multiplying by a constant k to express the result as a percent, per 1,000, per 100,000, or another convenient unit.
When to use it
Use this formula when you want a snapshot of disease burden, such as how many people are living with diabetes, asthma, or a mental health condition in a community. It is especially useful for planning healthcare services and comparing how common a condition is across groups or locations.
Worked example, step by step
Suppose a town has 80 people living with a condition out of a total population of 20,000. P = (Existing Cases / Total Population) × k P = (80 / 20,000) × 100,000 P = 0.004 × 100,000 P = 400 So the prevalence is 400 per 100,000 people.
Common mistakes to avoid
- using new or past cases instead of only people who currently have the condition
- forgetting to include the multiplier k when the answer needs to be reported per 1,000 or per 100,000
- confusing prevalence with incidence, which measures new cases over a time period
Did You Know?
A disease with a long duration will have high prevalence even if its incidence is low — diabetes is a classic example.
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More in Epidemiology & Public Health
View allIncidence Rate
BasicRate at which new cases of a disease appear in a population over a given period.
Basic Reproduction Number
IntermediateAverage number of secondary infections produced by one infected individual in a fully susceptible population.
Herd-Immunity Threshold
IntermediateMinimum fraction of the population that must be immune to prevent sustained spread.
Effective Reproduction Number
IntermediateActual average infections per case when part of the population is already immune.
SIR Model — Susceptible
AdvancedRate of change of the susceptible population in the SIR compartmental model.
SIR Model — Infected
AdvancedRate of change of the infected population — new infections minus recoveries.