# What is the prevalence?

Prevalence is a measure of how common a phenomenon is present within a population at a specific time point or within a specified time period. The impact of the prevalence on the Accuracy, the Positive Predictive Value and the Negative Predictive Value cannot be understated. As the prevalence increases, the PPV also increases while the NPV decreases. Analogously, as the prevalence decreases, the PPV decreases while the NPV increases — a mathematical explanation can be found here.

The prevalence is calculated as follows:

\frac{P}{P + N}