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Difference versus Ratio

Lead Author(s): Jeff Martin, MD

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%

Topic revision: r4 - 10 Jun 2009 - 15:15:29 - MaryB?

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