How to Calculate Relative Risk Reduction

Relative risk reduction is used in epidemiology to measure the reduction (or, possibly, increase) in risk from a particular treatment or condition. For example, one can calculate the relative risk reduction for lung cancer if someone quits smoking. It can be contrasted with measures such as absolute risk reduction. Relative risk reduction can be used in both observational studies and controlled trials.

Things You'll Need

• Calculator or computer

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Instructions

• 1

  Decide on control group and experimental groups or treatment groups. In a controlled trial, there will be a control group and an experimental group. In an observational study, there will simply be two groups (e.g., smokers and nonsmokers).

• 2

  Collect data on each group. This should include the number of events (e.g., death) and the number of non-events (e.g., people who did not die). Enter this into a two by two table, with one column for each group, and a row for event and a row for non-event.

• 3

  Calculate the marginal totals. That is, add the rows and add the columns. Because this is a two by two table, there will be two row totals and two column totals.

• 4

  Calculate the event rate for each column. This is the number of events, divided by the marginal total for that column. For the experimental group, this is called the EER (experimental event rate); for the control group, this is called the CER (control event rate). In an observational study, you can label these arbitrarily.

• 5

  Subtract the CER from the EER. Divide the result by the CER. This is the relative risk reduction.