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Estimates of deaths of bicyclists attributable to not wearing a helmet are corrected, with explanation
In 2006, colleagues and I published a study of US traffic deaths during 1982–2001.1 There were 858741 deaths, and we estimated that (a) 366607 could be attributed to alcohol use, (b) 259239 to not wearing a seat belt, (c) 31377 to lack of an air bag, (d) 12095 to not wearing a motorcycle helmet, and (e) 10552 to not wearing a bicycle helmet. Jointly, these factors accounted for 528105 deaths, 61% of all the traffic deaths; this joint total was less than the sum of the five counts because some deaths could be assigned to more than one factor.
To estimate the counts above, we used attributable fractions.2,3,4,5 Imagine a hypothetical series of 100 unhelmeted motorcycle drivers who died; how many of those deaths would have been prevented if all the drivers had worn a helmet? The risk of death in a motorcycle crash has been reported to be greater for an unhelmeted motorcycle driver than for an otherwise similar driver who was helmeted: risk ratio (RR) 1.54.6 The fraction of motorcycle driver deaths that can be attributed to not wearing a helmet is (RR − 1)/RR = (1.54 − 1)/1.54 = 0.35. So, among the hypothetical 100 unhelmeted motorcycle drivers who died, 100 × 0.35 = 35 deaths were due to not wearing a helmet.
A few months after the study of traffic deaths was published,1 I realized that I had made an error. To estimate deaths attributable to alcohol, no seat belt, no air bag, and no motorcycle helmet, I correctly used risk ratios for the outcome of death. But to estimate deaths of bicyclists resulting from not wearing a helmet, I used a risk ratio for brain injury, not death, when a helmet was not worn.7 Not all bicyclist deaths result from a brain injury. Whereas helmets may prevent some deaths from brain injury, it seems doubtful that helmets can prevent lethal chest or abdominal injuries. In retrospect, my mistake seems obvious; I cannot understand how I made it.
To fix my error, I searched for published estimates of the proportion of bicyclist deaths in vehicle collisions that were due to brain injury alone. Two studies were too small to be of use.8,9 One study of 4812 bicyclist deaths attributed 62% to a brain injury, but that estimate did not remove deaths that involved both a brain injury and other potentially lethal injuries.10
I therefore used US multiple cause of death public‐use data for the years 1991–1994.11 Among the 3015 bicyclists who died in a motor‐vehicle crash, 1727 (57%) had at least one code for head or brain injury, and 1058 (35%) had a code for head or brain injury and no code for other serious injury. The first of these proportions may approximate the upper limit for deaths due to a brain injury, whereas the second may estimate the lower limit. I averaged these to estimate the proportion of deaths that might have been avoided had there been no brain injury: (0.57 + 0.35)/2 = 0.46. I multiplied the attributable fraction of brain injuries that might be prevented by not wearing a bicycle helmet (0.65) by the proportion of bicyclist deaths that may be due to brain injury alone (0.46), to obtain a corrected attributable fraction for bicyclist deaths due to not wearing a helmet: 0.65 × 0.46 = 0.30.
My next chore was to re‐estimate all the statistics in the 2006 paper1 that required the bicycle helmet attributable fraction. This looked daunting: there were over 11500 files which filled 14 gigabytes of hard‐drive space, including hundreds of files to create or transform variables, carry out multiple imputation, and analyze the data. Fortunately, I was helped by some of my work habits.12 For a research project, I always create variables in one set of files, and perform analyses in another set. So I only needed to redo some of the analyses. And I always keep a computerized index of the analysis files, including a list of the electronic files of commands used to create each table and figure. I made copies of those command files, corrected the attributable fractions, and ran the statistical commands again. In a few days the work was done.
Corrected estimates are shown in abridged versions of tables 1 and 3. Part of fig 3 was also revised. Text on page 152 of the published paper1 reported deaths prevented by increased use of bicycle helmets: the published estimate was 239, the corrected estimate 73. The published estimate of deaths prevented by changes in all five factors was 293898, the corrected estimate 283412. I also corrected Appendix 4, which is available online from Injury Prevention.
When correcting a paper it is customary for authors to express regret for their errors. I feel regret for several reasons. Firstly, I do research in the hope that some of what I do may actually be useful in preventing future injuries; erroneous estimates cannot serve that purpose well. Secondly, I devote effort to finding and removing errors from my work before publication; it is discouraging and humbling when errors occur despite my precautions. However, I take some comfort from corrections; thanks to the cooperation of journals, it is possible to undo mistakes. I practiced medicine before starting a career in research; it is far easier to fix mistakes in research than in medical care.
Competing interests: None.