Interpreting Study Results Statistical and Clinical Significance

Although the WHI showed a statistically significant increase in the relative risk of CHD events among women who were randomly assigned to take HRT, it is important to consider the absolute difference in CHD events between the two groups to understand the strength of the association and to discuss the risk of HRT treatment with individual patients. Calculating absolute risk (in addition to relative risk) is a helpful way to understand the level of risk that HRT may add for a group of women who are at risk for CHD events (see Table 8-2).

In the WHI study, the relative risk of CHD for participants who took HRT was 1.29, with a 95% confidence interval that did not cross 1.0 (95% CI, 1.02 to 1.63). This figure (RR = 1.29) can generally be interpreted as HRT being associated with a 29% increase in CHD events. This summary measure was reported widely in medical journals and the mainstream press.

When reported in terms of relative risk, the weight of the association between HRT and CHD sounds ominous (i.e., a 29% increase). However, in terms of absolute risk attributable to HRT treatment, a less portentous picture emerges (see Table 8-2). In the WHI study, women taking HRT had an average rate of CHD events of 0.37% per year, an average of 37 events per 10,000 women each year, and those in the placebo group had an annual rate of 0.30%, or 30 events per 10,000 women each year. Although the adjusted RR of CHD is 1.29 (0.37 divided by 0.30), the attributable risk or risk difference between the two groups is 0.07% (0.37 minus 0.30). In other words, approximately seven additional cases of CHD occurred for 10,000 women using HRT during each year over the course of the study. The attributable risk of the treatment group can be summarized as the number needed to harm (NNH) or, if a study reports a beneficial effect, the number needed to treat (NNT). In this case the NNH was approximately 1430; on average, for every 1430 patients treated with HRT, one additional CHD event occurred (i.e., the inverse of the risk difference, 0.07, or 10,000 divided by 7) (see Table 8-2). The NNH or NNT is often a more understandable and useful summary of study outcomes when physicians and patients weigh the risks and benefits of a particular therapy.

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