What is Prevalence?

Definition

• Prevalence is the proportion of a population who have a specific characteristic in a given time period.

How is Prevalence Estimated?

• To estimate prevalence, researchers randomly select a sample (smaller group) from the entire population they want to describe. Using random selection methods increases the chances that the characteristics of the sample will be representative of (similar to) the characteristics of the population.
• For a representative sample, prevalence is the number of people in the sample with the characteristic of interest, divided by the total number of people in the sample.

                          # of people in sample with characteristic
Prevalence  =  ━━━━━━━━━━━━━━━━━━━
                               Total # of people in sample
 

• To ensure a selected sample is representative of an entire population, statistical ‘weights’ may be applied. Weighting the sample mathematically adjusts the sample characteristics to match with the target population.

How is Prevalence Reported?

• Prevalence may be reported as a percentage (5%, or 5 people out of 100), or as the number of cases per 10,000 or 100,000 people. The way prevalence is reported depends on how common the characteristic is in the population.
• There are several ways to measure and report prevalence depending on the timeframe of the estimate.
  • Point prevalence is the proportion of a population that has the characteristic at a specific point in time.
  • Period prevalence is the proportion of a population that has the characteristic at any point during a given time period of interest. “Past 12 months” is a commonly used period.
  • Lifetime prevalence is the proportion of a population who, at some point in life has ever had the characteristic.

How is Prevalence Different from Incidence?

• Incidence is a measure of the number of new cases of a characteristic that develop in a population in a specified time period; whereas prevalence is the proportion of a population who have a specific characteristic in a given time period, regardless of when they first developed the characteristic.
• Researchers may study incident (new) cases of illnesses to help identify causes and prevent additional cases. Incidence is often reported for infectious diseases.

How do Methods Impact Prevalence Estimates?

• A variety of methods are used to estimate prevalence of mental disorders.
• Differences in methodology may impact prevalence estimates. Some methodological differences that may affect comparisons between studies include, but are not limited to: the populations covered; the timing of data collection; sample design; mode of data collection; instruments and surveys used; operational definitions; and, estimation methods.