Relative or Absolute measures of effect (Pulse Article 2001)

This article is part of a series on Critical Reading.

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. For any trial that reports dichotomous outcomes (that is where patients can only be in one of two categories, such as dead or alive, pregnant or not) the results can be shown simply as a two-by-two table.

Non-pregnant Pregnant
Levonelle 976 11
Yuzpe 997 31

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.(2)

Each measure has its advantages and disadvantages. The relative risk of a given treatment (such as statins for the prevention of ischaemic heart disease) tends to be independent of the risk of the patients being treated. This makes it a good measure to use when combining the results of different trials in a meta-analysis (3).

Risk difference on the other hand is helpful when considering treatments for individual patients as the amount of difference a treatment will make to them depends on their level of risk. A good example of this comes from the comparison of different oral contraceptive pills in terms of the risk of deep vein thrombosis. Safety studies have indicated that third generation oral contraceptive pills carry twice the risk of venous thromboembolism as the older pills, this is a relative risk of 2 and caused a great deal of alarm amongst pill takers. However the absolute risks are very low, 1 in 10,000 with the older pills and 2 in 10,000 with the third generation ones; the risk difference is 0.0001. Put another way 10,000 women would have to take the third generation pills for one year before one of them suffered thromboembolic disease as a consequence giving a Number Needed to Harm (NNH) of 10,000.

Of course the interpretation of Numbers Needed to Treat may be dependent on how important the consequences are and some women opted to change pill to minimise their risks whilst others were happy to continue, as the individual risk to them was so low.

Those of you who are interested in seeing some examples of graphical displays of Numbers Needed to Treat in different clinical scenarios related to primary care will find examples in the Cates plots in other articles on this site (such as Vitamin D for asthma).

There are related article on this topic (Relatively Absolute and Communicating Risk).


1. Task Force on Postovulatory Methods of Fertility Regulation. Randomised controlled trial of levonorgestrel versus Yuzpe regimen of combined oral contraceptives for emergency contraception. Lancet 1998;352:428-33.

2. Which postcoital contraceptive? Cates C. BMJ 2000;321:664

3. Egger M, Davey Smith G, Phillips AN. Meta-analysis: principles and procedures. BMJ 1997; 315: 1533-1537.