Crook and Feikh

Adrian D Cook and Aziz Sheikh are helmet zealots. They published a paper claiming that cycle helmets reduce head injuries by 60%, but in calculating this they made a schoolboy error. As pointed out by James Annan in a letter to the publishing journal:


 * 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.
 * 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.

Their subsequent refusal to admit that this basic error invalidated their conclusions led to them being derided on uk.rec.cycling, where they are still referred to as "Crook and Feikh".