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 Vol. 17 No. 6, June 2007
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Right arrow Weighing the Evidence

AAP Grand Rounds 17:64 (2007)
© 2007 American Academy of Pediatrics

EPIDEMIOLOGY

Weighing the Evidence: Relatives, Absolutes, and the Number Needed to Treat

Michael Aldous, MD, MPH, FAAP
The first 20% of the full text of this article appears below.

Interpretation of "significant" research findings can be challenging. Outcome differences can be reported in relative or absolute terms. Belshe et al reported that children who received live attenuated influenza vaccine had a 3.9% risk of influenza during the subsequent season, while children who received inactivated influenza vaccine had an 8.6% risk. This difference in influenza rates can be expressed as the absolute risk reduction (ARR), also known as risk difference, which is simply the arithmetic difference between the outcome rates in the treatment and comparison groups (ARR = 8.6% – 3.9% = 4.7%).

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