Most of the things done by the professionals in the health service have evolved over the years through personal experience, expert views and trial and error. Since the last World War, initially with the randomized controlled trials of TB therapy, there has been increasing interest in objectively measuring the effect of treatment on patients. The proponents of this approach suggest that, where tested, personal experience, even in those with a great deal of experience, does not always allow us to draw our evidence from sufficient numbers nor does it immunize us against unfounded enthusiasm for one treatment over another.