3. Prevalence

In a population of 10,000 patients having a certain disease, 50 persons are reported to be affected by another disease in addition. This is called a ‘co-morbidity’. So what is the prevalence of the co morbidity in this population?

The mathematical way to calculate this would be:

Number of cases divided by the population. The result multiplied by 100

This formula will provide us with the information as a percentage. By dividing 50 by 10,000 and multiplying the result by 100 (to make it a percentage) we find out that 0.5% of the population is affected. So the prevalence of the co-morbidity is 0.5% in our population.

Apply the formula with 50 cases and population 10.000. Result 0.5%

Rather than expressing prevalence as a percentage, we can describe it as the number of people affected in a standard sized population, for example 1,000 people. So instead we would calculate:

Apply the formula with 50 cases and population 10.000. Standard size population of 1000. Result 5

This means that, for each 1,000 patients, five of them have co-morbidity.

Keep the example of the party in mind. Prevalence is like describing a group photo:
  • How many people can you see there? That number is your population.
  • How many people share a certain feature (e.g. same hair colour)? This number is used to calculate prevalence.