Statistical Measures

Relative Risk & Odds Ratio

Odds Ratio (OR)

The RR can only be calculated if incidence data are available. The Odds Ratio (OR) may be used in situations where we do not have denominator data to calculate incidence rates.

• The odds ratio is used frequently in case/control studies, which we will cover in more detail in the workshop portion of this course.
• In a case/control study, ill persons’ characteristics and exposures are compared with those of well persons (“controls”) selected from the same population in which the outbreak occurred.
• Example: in an outbreak suspected to stem from exposure to contaminated food at a restaurant, the ill persons’ food selections could be compared with those of some well people who also ate at the restaurant the same day. This could be done even if we didn’t know exactly how many people ate at the restaurant that day.

• A two-by-two table is constructed, just like the one used to calculate RR, with the same letters (a-d) used to label the cells. The OR is calculated by multiplying across the cells.