We would like to point out a practice that is becoming widespread in reporting of associations. Odd ratios (OR) are commonly converted to percentages presumably for the benefit of the lay audience. Due to the substitution of odds for probabilities and the asymmetrical distribution of protective and risk ORs, this practice leads to misleading reporting. This is particularly troubling in genetic association studies where associations may be reported as a risk association with one allele of a biallelic single nucleotide polymorphism or a protective association with the other allele. These two versions of the same association are identical in magnitude, but the current practice results in reporting different effect sizes in percentages. Here, we suggest that direct conversion of ORs to percentages should be discontinued, and we also remind that there are useful transformations readily available as replacements.

Effect size in retrospective case-control association studies is customarily reported as an odds ratio (OR). ORs lie between zero and infinity, with unity indicating no association. For protective associations, the OR is between 0 and 1. Risk associations yield an OR of greater than 1, the upper limit being infinity. Since many people think more easily in terms of probabilities than odds, ORs are often converted into percentage changes in risk, especially in press releases. It is now common practice to state a protective OR of 0.6 as 40% reduction in risk, assuming a rare disease. The reciprocal risk association with the alternative allele yields an OR of 1.67 and this would be reported as a 67% increase in risk. On the other hand, a protective association with an OR of 0.60, and a risk association with an OR of 1.40 would be reported as 40% decrease and increase, respectively, in risk. The impression given is that these two associations in opposite directions are similar in magnitude, but actually they are not. Finally, the notion of fold-change and percentage increase are often intertwined, so OR=3.0 is transformed to a 300% increase in risk, but OR=2.0 to a 100% increase. In each of these scenarios, conversion of ORs to percentages leads to erroneous reports.

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