Relative or absolute measures of effects

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  • Format Texts
  • Language/s English
  • Target Audience Further education
  • EBM Stage 3 - Appraising evidence
  • Duration <5 mins
  • Difficulty Introductory

Key Concepts addressed


Measuring outcomes in clinical trials can be done in a variety of ways, and presentation of the results may influence the way that readers respond.

The data shown indicates that 1% of patients given Levonelle-2 for post-coital contraception become pregnant in comparison with 3% of those who are given the older Yuzpe method (1). This can be reported in different ways. The Relative Risk of becoming pregnant is obtained by dividing the risks of pregnancy in the treated and untreated groups and comes out as 0.33 if you have Levonelle-2 (or in other words your risk of becoming pregnant is one third of the risk with Yuzpe). This sounds impressive in comparison to the Risk Difference which is obtained by subtracting the risk in the two groups and is only 0.02 because the pregnancy rate is low in both groups. The Number Needed to Treat (NNT) with Levonelle-2 rather than Yuzpe to avoid one extra pregnancy is the inverse of the Risk Difference and in this case works out as 63 patients.

View the article here

From Dr Chris Cates’, EBM Website.


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