How To Calculate Incidence & Prevalence

Incidence and prevalence are statistics widely used for reporting on diseases. "Incidence" is how quickly new cases are occurring; "prevalence" is how much of the population is affected. The calculations can be used for conditions other than medical reporting; for instance, we could talk about the prevalence of students dropping out of high school, or the incidence of getting a driver's license.

Step 1

Define the population at risk. This should be a group of subjects (e.g. people) who are currently affected.

Step 2

Take a random sample from that population. This may be impossible to do precisely, which can lead to biases in the estimation of incidence.

Step 3

Follow that sample for a given time.

Step 4

Check the status of the sample periodically. A certain number will be affected over the course of time.

Step 5

Calculate the subject years at risk. Essentially, this means: Each subject is at risk for a given time; add up these times and convert to years. That is subject years at risk.

Step 6

Divide the number of subjects who got the condition by the subject years at risk. That is the estimate of the incidence rate.

Step 1

Define the population. This should be the entire population, whether they have the condition or not.

Step 2

Take a random sample of this population.

Step 3

Find how many subjects in the sample have the condition.

Step 4

Divide the number in step 3 by the number in step 2. This is the estimate of the prevalence.