Incidence Rate
Rate at which new cases of a disease appear in a population over a given period.
Formula
Variables
Example
200 new cases in 50 000 at risk → IR = 400 per 100 000
How it works
Incidence rate tells you how quickly new cases are appearing in a population that could actually become ill. It compares the number of newly diagnosed cases to the size of the population at risk, then scales the result by a multiplier such as 1,000 or 100,000 so the rate is easy to read and compare. This works because the numerator counts only fresh cases during the time period, while the denominator represents the group that could have developed the disease during that same period.
When to use it
Use this when studying how fast an infectious disease, chronic condition, or injury is emerging in a defined population over a specific time period. It is especially useful for comparing disease occurrence between places, groups, or years.
Worked example, step by step
Suppose a town has 12 new cases of measles in one year among 8,000 people at risk. IR = (New Cases / Population at Risk) × k IR = (12 / 8,000) × 100,000 IR = 0.0015 × 100,000 IR = 150 Incidence rate = 150 new cases per 100,000 people per year
Common mistakes to avoid
- Using total existing cases instead of only new cases
- Including people who already had the disease in the population at risk
- Forgetting to multiply by the chosen k, which makes the result hard to interpret
Did You Know?
Incidence tells you the speed at which a disease is spreading — prevalence tells you how much of it already exists.
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More in Epidemiology & Public Health
View allPrevalence
BasicProportion of a population that has a particular condition at a point in time.
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.