I'm using bias in a non-pejorative, technical, statistical sense. Clearly either: a) There is some underlying bias why studies where helmets were less effective were also less likely to meet Thompson's Cochrane review standards OR b) There is a bias in the Cochrane review OR c) There is a "biased" co-incidence (random chance that creates a pattern that looks like bias).
FWIW, I have both studies open. They both have good discussions on this. I think we'd both be better off just reading them and deciding from that. I'm sticking to my belief that bias is best dealt with by gathering and aggregating more data and using mathematical tools to deal with any differences in methodology and results, rather than using more subjective "quality" criteria to exclude data-points.
One thing, the Elvik meta-study a different data-set to the Thompson Cochrane Collab. paper, which is interesting. The Elvik paper is updating a 2001 meta-study, Attewell, with new data-points. The Cochrane paper uses some of the same, and additional ones. There are primary-papers in the Attewell study though which the Thompson paper did not find at all - even though it found the Attewell meta-study. Similarly there are primary-papers in the Thompson paper, which pre-date Attewell significantly, but which Attewell does not mention. This makes me think the search strategy in both those meta-studies might have been sub-optimal - unless there's some important factor I've missed. If I havn't missed anything, it'd be interesting to see a meta-study with search criteria that caught at least all the primary works in those meta-studies.
Also worth noting is that several of the studies excluded from the Thompson paper were published in Accident Analysis & Prevention, as was the Attewell meta-study.