Since it is a ratio of ratios, the odds ratio is very difficult to interpret. The relative risk is easier to interpret, so the odds ratio alone is not very helpful. However, there are certain commonly occurring situations in which the estimate of the relative risk is not very good and the odds ratio can be used to approximate the relative risk of the event of interest. The odds ratio should be used as an approximation to the relative risk of the event of interest when both of the following conditions are met:

- The probability of the event of interest is small (<0.1). This condition guarantees that the odds ratio will make a good approximation to the relative risk. In this example, the event of interest is a response to the mailing.
- The design of the study is case control. This condition signals that the usual estimate of the relative risk will likely not be good. A case-control study is retrospective, most often used when the event of interest is unlikely or when the design of a prospective experiment is impractical or unethical.

The first condition is met in this example, since the overall proportion of respondents was 9.6%. However, the design of the study was not case control, so it's safer to report 1.635 as the relative risk, rather than the value of the odds ratio.