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Old June 10th 04, 12:59 PM
James Annan
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Default Fundamental error in "Trends in serious head injuries..." Cook andSheikh 2003Fundamental error in "Trends in serious head injuries..." Cookand Sheikh 2003

I sent the following to "Injury prevention on-line" over a week ago, but
it shows no sign of being published and my follow-up email has not been
answered. I guess someone might as well see it, typo and all. Isn't the
internet great for vanity publishing?

Neither Cook nor Sheikh replied to my email, either.

Their original article, to which this refers, can be found on

http://ip.bmjjournals.com/cgi/content/full/9/3/266

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"Fundamental error in "Trends in serious head injuries..." Cook and
Sheikh 2003"
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!-- article ID: 9/3/266 --

P The main conclusion of Cook and Sheikh (2003), that a bicycle helmet
prevents 60% of head injuries, is incorrect due to a fundamental error in
the way they have treated their percentages. A correct analysis
demonstrates unequivocally that there must be major confounding factors in
their data set that they have failed to take into account, and therefore
any estimate of helmet effectiveness is purely speculative.

P Assuming that their basic analysis of the data is correct (although
the numbers they quote in the text do not actually appear to match the
figure plotted), they arrive at a figure of a 3.6% for the reduction in
the head injury (HI) rate for cyclists, over and above the "background"
reduction that pedestrians have also seen. They assume that this drop in
HI is due to increased helmet-wearing. However, this reduction is
presented in terms of the number of percentage points, and relative to the
baseline value of 27.9% HI for cyclists in 1995-6 it actually represents a
3.6/27.9 = 13% drop in the HI rate.

P The decrease in the number of helmetless cyclists over the same
interval is 5.8 percentage points from a baseline of 84% unhelmeted,
giving the percentage drop as 5.8/84 = 7%. Cook and Sheikh calculate
helmet effectiveness to be given by the ratio 3.6/5.8 = 60%. However the
correct expression to use is 13/7 = 186%. In other words, "helmet
effectiveness" is so high that each helmet does not just save its wearer,
but a non-wearer too. At this rate, head injuries would be eliminated
completely if just a little over half of all cyclists wore them! This is
clearly ludicrous.

P A more reasonable conclusion to draw from this would be that there
are some other factors that are responsible for the large drop in HI rate,
and therefore any attempt to attribute some part of the total 30%
(8.49/27.9) change to the provably marginal impact of a very small number
of extra helmet wearers is at best highly speculative and fraught with
inaccuracy.

P What makes this all the more poignant is the fact that the authors
have recently produced a book entitled "Basic skills in statistics"!

P James Annan

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