The ‘20%’ is a basic statistical error promoted by a misleading abstract and press release from JAMA Internal Medicine – associated with the Journal of the American Medical Association, a (supposedly) reputable source. The authors estimated an ‘odds ratio’ of 1.19 for muscular-skeletal problems, which the Daily Mail interpreted as a 20% increased risk. I’m afraid we need to get a bit technical now. An odds ratio is a standard measure that statisticians and epidemiologists (yes, them again) use to measure an association between an exposure (here statins) and an event (muscle problems). It is defined as the odds of the event given the exposure, divided by the odds without the exposure. The crucial thing is the use of odds, not risk, where odds is the probability of the event divided by the probability of the event not occurring (why statisticians should use this bizarre measure is another story – see for example this Wikipedia description).

Table 4 of the paper (not reported in the abstract) reports risks with and without statins of 87% vs 85%, which translate to odds of 0.87/0.13 = 6.7 and 0.85/0.15 = 5.7. The odds ratio is therefore 6.7/5.7 = 1.18 (their figure of 1.19 involved some adjustment for other factors). Alternatively, the risk ratio was 0.87/0.85 = 1.02, a 2% relative change, while the difference in absolute risks was 0.87 – 0.85 = 2%. The Code of Practice for the British Pharmaceutical Industry has banned the reporting of relative risk without also giving the change in absolute risk. Why this is still considered acceptable within epidemiological papers is beyond me.