A case-control study of the effectiveness of bicycle safety helmets

Thompson RS, Rivara, Thompson DC. The New England Journal of Medicine, 1989.

This paper has been widely quoted in support of helmet wearing, and also widely criticised by those who see its methodology as flawed.

The authors acknowledged two sources of uncertainty: statistical error due to the fairly small sample, and bias in the sample: "We cannot completely rule out the possibility that more cautious cyclists may have chosen to wear helmets and also had less severe accidents".

Criticisms

• The study is non-randomised. [various]

• Cyclists wearing helmets were mostly white and riding in parks or on cycle paths accompanied by adults. Cyclists without helmets were more often black or other races, riding alone on city streets. [D Robinson, M McCarthy]

• The control population had a helmet wearing rate nearly 7 times greater than that confirmed by a concurrent study for children in the Seattle area as a whole. See: Head Injuries and Bicycle Helmet Laws [D Robinson]

• The same methodology can be used on other data in the study to show that helmet use also reduces the risk of injury to other parts of the body by 72%. [P van Schaik]

• The study does not distinguish facial injuries from other head injuries, although helmets would not prevent the former. If facial injuries are excluded, Thompson's 85% reduces to 61%, but the number of cyclists wearing helmets is too small for this to be statistically significant. [McDermott et al, Journal of Trauma]

• In further re-working of Thompson's data, McDermott found that only 40% of head injuries would be reduced using approved helmets, though injury rates increased for the neck, extremities and pelvic region.