Incidence & Prevalence

Usually the first question about a certain health event will be: How many people are affected? Or what is the prevalence?

Depending on the population we are looking at, the answer will differ. Let’s say that there are 10,000 cases in country A and 20,000 cases in country B. Could we state that the health problem is far more serious in country B? Let’s have a look on the following table:

	Country A	Country B
Persons affected by a certain health event	10.000	20.000
Total population of the country	20.000	200.000

In country A, a total of 10,000 out of a population of 20,000 are affected, which means that 50% are affected by the health event.

In country B, a total of 20,000 out of a population of 200,000 are affected, which means that 10% are affected by the health event.

The mathematical way to calculate this would be:

With this formula we get information about the percentage of a population that is affected by a health event.

While our first impression would be that 20,000 are twice as many as 10,000 (which is definitely true!), we now have a very different picture, as we put the numbers in relation to the population.

Understanding these basics will make the concept of prevalence easier to understand. The prevalence compares the number of persons having a certain characteristic with the total population.