2. Prevalence - How many people are affected?

1. Ways to measure and report prevalence

To estimate prevalence, researchers randomly draw 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 and allow extrapolating the results to the population.

Bear in mind that in the nomenclature the term “population” is commonly used but in reality, mostly means a representative sample from a population.

There are several ways to refer to prevalence depending on the timeframe of the measurement:

  • Point prevalence: the proportion of a population that has a health event at a particular point in time. It includes all previous cases who still have the condition and are still members of the population. A good way to think about point prevalence is to imagine that you took a snapshot of the population and determined the proportion of people who had the condition of interest at the time the snapshot was taken. For example, in a survey you would be asked if you are currently smoking.


  • Period prevalence: is similar to point prevalence, except that the "point in time" is broader. It is the proportion of a population that has a health event during a given time period, often 12 months. For example, in a survey you would be asked if you have smoked during the past 12 months.