Absolute vs. Relative Risk

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Radio commentator Paul Harvey has a feature called “The Rest of the Story.” Researchers, clinicians or journalists who report only on relative differences in making claims about a new idea should tell the rest of the story. It is absolute differences that probably matter most to most people trying to make sense out of such claims.

Consider the risk for blindness in a patient with diabetes over a 5-year period. If the risk for blindness is 2 in 100 (2%) in a group of patients treated conventionally and 1 in 100 (1%) in patients treated with a new drug, the absolute difference is derived by simply subtracting the two risks: 2% – 1% = 1%.

Expressed as an absolute difference, the new drug reduces the 5-year risk for blindness by 1%.

The relative difference is the ratio of the two risks. Given the data above, the relative difference is:

1% ÷ 2% = 50%

Expressed as a relative difference, the new drug reduces the risk for blindness by half.

Each is accurate. But if your job is marketing manager for the new drug, you are likely to only use the relative risk reduction. If your job is journalist, you would serve your readers and viewers better by citing the raw data and pointing out the absolute risk reduction. That’s the “rest of the story” often missing in news releases and direct-to-consumer prescription drug ads.

For more on this topic, see Gil Welch’s column, “The Problem is Relative.”


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