EDITOR,—Charles R Gillis and David J Hole provide clear evidence that patients of specialist surgeons have a higher survival rate from breast cancer than those of non-specialist surgeons.1 However, they go on to conclude that this association is one of cause and effect, rather than due to differences in the cases referred to the two groups of surgeons (case selection bias). Their reason is that the difference in outcome persisted after adjustment for some prognostic factors; but statistical techniques can correct …