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".