Author: Lauren Bishop

Ratios, proportions, and rates are all used to measure frequencies of health and disease.

All three frequency measures compare one part of the population to either another part of the population, or to the entire population.

Ratio	A comparison of health event numbers or rates between groups.
Proportion	A comparison of a part to the whole, or a type of ratio where the value of the numerator is included in the denominator.
Rate	A measure of change in one quantity for each unit of another quantity.

Ratio

A ratio is calculated by dividing, e.g., the number or rate of health events in one group by the number or rate of health events in a second group.

Ratio results are often written as the result “to one” or “result:1”.

Example

56	345

Let us assume that we are reviewing results from a study of bovine spongiform encephalopathy (BSE, or Mad Cow Disease). What was the ratio of non-infected versus infected cows? The ratio of non-infected to infected cows = 345/56 × 1 = 6.2:1. In other words, for every infected cow, there are ~6.2 non-infected cows.

Proportion

Calculating a proportion is simply dividing, e.g., the number of persons or health events by the total of persons or health events.

In this way, the numerator is a subset of the denominator.

Example

56	401

Let us return to our cows and calculate the proportion of infected bovines in the BSE study. There are still 56 infected cows, but now the denominator is the total number of cows in the study (56 infected + 345 non-infected = 401 cows). Of the 401 cows in the study, 56 were infected with BSE. The proportion of infected cows: 56/401 = 0.14 = 14%

Note Proportions are often used as descriptive measures in epidemiology. Specific proportions (e.g., the incidence proportion) will be discussed later in this section.

Rate

In epidemiology, a rate is the frequency with which a health event occurs within a specific population at or during a specific time or time interval. In other words, a rate is a measure of the risk of the health event.

Even more specifically, it is the instantaneous risk that the health event will occur at the given time point.

In epidemiology, the change in one quantity for each unit of another quantity is often reported in terms of changes in the quantity of health events for units of time. For an example of a rate calculation, see “incidence rate” in Mortality.