We apologise for an electronic glitch that affected the references in this paper by Nicky Britten and colleagues (19 February, pp 484-8). Unfortunately, at a late stage in the editorial process the reference numbers in the text disappeared, and this went unnoticed. We have reinstated the numbers in our website version; readers may access the corrected article at www.bmj.com/cgi/content/full/320/7233/484.