How to Calculate Incidence & Prevalence

••• Witthaya Prasongsin/Moment/GettyImages

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.

How to calculate incidence

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

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

    Follow that sample for a given time.

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

    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.

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

How to calculate prevalence

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

    Take a random sample of this population.

    Find how many subjects in the sample have the condition.

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

Related Articles

How to Calculate Crude Birth Rate
How to Calculate Sharpe Ratio
How to Calculate a T-Statistic
How to Calculate Statistical Sample Sizes
How to Calculate a Demographic Percentage
How to Calculate a Growth Trend
How to Compute a Population Mean
How to Find a Sales Trend Percent
How to Estimate the True Proportion
How to Calculate NAIRU
How to Calculate Reliability & Probability
How to Calculate the Confidence Interval of the Mean
How to Determine Sample Size With Mean & Standard Deviation
How to Calculate the Rate of Decay
How to Calculate Statistical Mean
How to Calculate the Coefficient of Variation
How to Calculate Variance From Standard Error
How to Calculate a P-Value
Advantages & Disadvantages of Finding Variance
How to Find the Midpoint of the Interval