# Difference versus Ratio

## Ratio versus Difference in Cohort Study

There are two basic ways to compare measures of association in a cohort study:
• Ratio: form a ratio of one over the other
• Difference: subtract one from the other
Although comparing two incidence measures by a ratio or by a difference uses the same numbers, they convey different information and are used for different purposes.

Can take the difference of either an incidence or a prevalence measure but rare with prevalence.

## Ratio Is a Relative Measure

A ratio measure is known as a relative measure since it is telling you how large the incidence is in one group relative to the other.
• It tells you nothing about the absolute difference between them.
For example, a risk ratio of 2.0 could be obtained by a cumulative incidence of
• 4% versus 2%
• 20% versus 10%
• 80% versus 40%
• and many other combinations.

## Difference Is an Absolute Measure

Risk difference is often called an absolute measure, as it conveys how much the one group differs from the other in the scale of 0 to 100%.

## Example of Difference Versus Ratio

Example using incidence:

Cumulative incidence 26% in exposed and 15% in unexposed
• Risk Ratio: 0.26 / 0.15 = 1.7
• Risk Difference: 26% - 15% = 11%