Measure of Association - Rate Ratio

Definition of Rate

In a cohort study:

Rate is based on events per person-time = incidence rate

We have limited the use of the word rate to measures of incidence based on person-time rates,

Definition of Rate Ratio

Rate ratio = ratio of 2 incidence rates = relative rate

If we have limited the use of the word rate to measures of incidence based on person-time rates,

Example: Comparing of Two Person-time Rates with a Ratio

Ratio of two person-time rates In the Ray paper, the following data were given: This is the example of forming incidence rates in a cohort with a time varying exposure, in this case use of non-steroidal anti-inflammatory medications (NSAIDS).

The basic measure of association reported was the rate ratio, A rate ratio is simply the ratio of two rates It should be clear that an incidence rate should not be compared to cumulative incidence: In comparing two incidence rates, the important thing to remember is that the time units of the two measures forming the ratio must be the same. The authors correctly reported their measure of association as a rate ratio.

Example: Rate Ratio Described as Relative Risk

Cancer mortality rate ratio by BMI group Calle 2003:

  1. 6 per 100,000 person-years ( BMI 40+)
  2. 3 per 100,000 person-years ( BMI < 40)

Described as relative risk in the article (very common practice)

Although we think everyone should use the rate ratio (relative rate is a defensible alternative since the term relative implies a ratio),


E. E., Rodriguez, C., Walker-Thurmond, K., & Thun, M. J. (2003). Overweight, obesity, and mortality from cancer in a prospectively studied cohort of U.S. adults. N Engl J Med, 348 (17), 1625-1638

W. A., Stein, C. M., Daugherty, J. R., Hall, K., Arbogast, P. G., & Griffin, M. R. (2002). COX-2 selective non-steroidal anti-inflammatory drugs and risk of serious coronary heart disease. Lancet, 360 (9339), 1071-1073.

Jeff Martin, MD

-- MaryB - 10 Mar 2009