Research Article
Three is the magic number.
Br Med J (Clin Res Ed) 1982; 284 doi: https://doi.org/10.1136/bmj.284.6308.23 (Published 02 January 1982) Cite this as: Br Med J (Clin Res Ed) 1982;284:23This is a PDF-only article. The first page of the PDF of this article appears above.