EDITOR-Relative risks are often reported incorrectly in medical journals. In a paper in the BMJ, Jian-Min Yuan and colleagues describe a relative risk of 3.72 for the relation between cancer of the upper aerodigestive tract and heavy drinking as a "3.7-fold increased risk"; this description is incorrect.(1) They also describe a relative risk of 1.30 for total mortality and heavy drinking as "a 30% excess risk"; this description is correct. They also write that "heavy drinking was associated with a significant 1.7-fold excess in risk of death from stroke"; the excess is 70%.
In an earlier paper by the same authors in JAMA there were similar problems. (2) A relative risk of 1.6 was correctly described as a "60% greater risk;" while a relative risk of 2.3 was incorrectly described as a "2.3-fold excess risk." The paper also stated that "rates in Shanghai Chinese were 2-fold to 8-fold higher than in Los Angeles whites;" but the rates in Shanghai were actually two to eight times those in Los Angeles whites. Such problems were not confined to the interpretation of relative risks. The sentence "In China, the yearly per capita consumption of cigarettes has increased 3-fold between the 1950s and 1987, from about 500 to 1748" is incorrect; the increase is actually twofold or 200%. Peto et al, in an accompanying editorial, stated "that heart attack mortality is five times lower, and that stroke mortality is five times higher."(3) This was a problem because the authors were referring to the ratio of 5.3 (366/69) and 1:4.2 (48/201) respectively.
I have found similar problems in reports by American, British, and Chinese authors (in alphabetical order). This problem is important when relative risks or differences of two measures are described. We have to be cautious about the confusing meaning of the suffix "-fold"; n-fold is equal to n times, and is equal to n x 100%. Therefore, a relative risk of 3.5 is 2.5-fold, or 2.5 times, or a 250% increase or excess in risk, not 3.5-fold or a 350% increase or excess. I wonder how long we will continue to see such inadvertent inflation of relative risks in the literature.
T H Lam, Professor, Department of Community Medicine and Unit for Behavioural Sciences, University of Hong Kong, Hong Kong
- Yuan J-M, Ross RK, Gao Y-T, Henderson BE, Yu MC. Follow up study of moderate alcohol intake and mortality among middle aged men in Shanghai, China. BMJ 1997;314:18-23 (4 January).
- Yuan J-M, Ross RK, Wang X-L, Gao Y-T, Henderson BE, Yu MC. Morbidity and mortality in relation to cigarette smoking in Shanghai, China: a prospective male cohort study. JAMA 1996;275:1646-50.
- 3. Peto R, Chen Z, Boreham J. Tobacco-the growing epidemic in China. JAMA 1996;275:1683-4.
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