Statistical Measures

Relative Risk & Odds Ratio

The last two types of frequency measures we will study are relative risk (also called risk ratio) and odds ratio. These statistics are used in outbreak investigations and will be discussed again in the workshop portion of this course.

Relative Risk or Risk Ratio (RR)

• Compares the risk of disease or death in two groups.
• The two groups may be defined by a demographic factor (for example, male vs. female).
• More commonly, they are defined by a difference in their exposure to a suspected risk factor for disease (for example, ate the potato salad or didn’t).
• Often, the group of primary interest is labeled “exposed,” and the comparison group is called “unexposed.”
• The group of primary interest goes into the numerator, and the comparison group is the denominator.




• “Risk” is defined as an incidence rate or attack rate of the disease in each group. To calculate RR, a two-by-two table is set up as shown on the next screen: