Re: Bicycle helmets and the law
Goldacre and Spiegelhalter state that the Dennis study “has somewhat superior methodology – controlling for background tends and modelling head injuries as a proportion of all cycling injuries”. However, several population level helmet law studies have controlled for background trends and included both head and non-head injuries, and shown that the effect of the legislation on hospital admissions for cycling head injuries to be far from minimal:
- Carr/MUARC (1995), http://www.monash.edu.au/miri/research/reports/muarc076.html (Victoria, Australia)
- Hendrie (1999), http://www.ors.wa.gov.au/Documents/Cyclists/ors-cyclists-report-helmets-... (Western Australia)
- Povey (1999), http://www.ncbi.nlm.nih.gov/pubmed/10487351 (New Zealand)
- Scuffham (2000), http://www.ncbi.nlm.nih.gov/pubmed/10487351 (New Zealand)
- Karkhaneh (2006), https://era.library.ualberta.ca/public/view/item/uuid:432ec921-cf50-4b91... (Alberta, Canada)
- Walter (2011), http://www.ncbi.nlm.nih.gov/pubmed/21819836 (New South Wales, Australia)
The head injury results in all these population-level longitudinal studies, and the AIS3/4 head/brain injury results in the Carr study, are consistent with the (hospital control) results of the Thompson Cochrane Review, and the Attewell and Elvik meta-analyses, of case-control studies.
Dennis et al. cite the Scuffham and Walter studies as being "limited by sample size or methodological quality". However both the Scuffham and Walter analyses took baseline trends into account, and had (more than) adequate sample sizes.
The Walter study had 18 (monthly) time points before the New South Wales helmet law, and 18 after the law. The Dennis study had a total of 15 (yearly) time points.
In the case of Ontario (30% of the 1994 injuries), where the helmet law was Oct 95, there was only a single true pre-law time point. Each year of data is April to March, so the ‘1995’ data, which was treated as pre-law, actually contained both pre- and post-law data.
For British Columbia (19% of the 1994 injuries), the helmet law was Sep 96, Dennis et al. treated 1996 as the year the intervention occurred. However, they note that the Canadian cycling season runs to September, so 1996 would have been treated as post-law, when in fact much of the data would have been pre-law.
Collinearity between 'time' and 'time since intervention' may also have contributed to the “minimal effect” finding.
Macpherson claimed that the Povey and Scuffham analyses, and a preliminary (1992) MUARC study by Cameron, "failed to include a concurrent control group in the analysis"; however all 3 analyses used cyclist non-head injuries as concurrent control groups. (Povey's and Scuffham's analyses also included non-cyclist injuries.) Dennis also cites the preliminary 1992 Cameron/MUARC study; both Macpherson and Dennis have apparently overlooked the (1995) Carr/MUARC study (4 years of post-law data), which superceded the (1992) Cameron study (1 year of post-law data).
This (2013) paper debunks the Fyhri and Walker risk compensation, and Robinson safety in numbers, claims: http://acrs.org.au/wp-content/uploads/26_Olivier_PR.pdf also see http://injurystats.wordpress.com/author/jakeolivier/).
Competing interests: No competing interests