# Measurements of disease frequency

Now that we have covered the basic demographic measures used in epidemiology, we will learn about measures used to describe disease and health status. These are central to epidemiology and it is important to understand how each is derived. First, we will discuss measurements used in epidemiology to describe disease occurrence: ratios, proportions, and rates.

A ratio is one number divided by another, where the two numbers are not necessarily related. For example, the sex ratio describes the number of males divided by the number of females, which are not part of the same group.

A proportion is also one number divided by another, but the numbers are related such that the numerator is a subset of the denominator. For example, the proportion of women who are black is the number of black women divided by the total number of women in a population. Proportions are typically described using percentages and all the individual parts add up to 100.

A rate is one number divided by another where time is part of the denominator. For example, travel speed in miles per hour is a rate. You may notice that some “rates” in texts and in our discussion do not have a time component as central to the equation; this is unfortunately simply a common misuse of the term. Therefore, it is very important for you to understand what each calculation or measure is describing so you do not misinterpret analyses based on tricky terminology.

Now we will learn about the common measures of disease frequency.