THE CLINICAL EVALUATION OF ANTIDANDRUFF SHAMPOOS 453 MR. VAN ABBI•: We have carried out some experiments. We did not like the Hair Vac (as used by Vander Wyk) in its original state but even when this had been modified by us, we still felt that it was impossible to know whether the course of the con- dition was being influenced. This is the main difficulty, for one just cannot know and therefore it does not seem to be a desirable technique. MR. R. CLARK: Did you run a "vacuum cleaner" trial in conjunction with your own subjective method of assessment? MR. VAN ABBi: We have not done so yet, although we have considered the possibility.

J. Soc. Cosmetic Chemists 18 455-467 (1967) ¸ 1967 Society of Cosmetic Chemists of Great Britain The evaluation clinical trials of placebos in P. MACDONALD* Presented at the Symposium on "Product Testing", organised by the Society of Cosmetic Chemists of Great Britain in Eastbourne, Sussex, on 15th November, 1966 Synopsis--A model is proposed for the response of living organisms to the action of drugs when placebo reaction is likely to occur. The estimation of the parameters in the model is discussed together with tests of hypotheses about the parameter values. INTRODUCTION The clinical evaluation of drugs and medicaments is as old as medicine itself but it is comparatively recently in the history of the subject that the actions of various drugs have been recorded and compared in a quantitative and scientific fashion. The value of a treatment can only be expressed in meaningful terms by reference to the state of affairs when the treatment is not used, either because of an alternative treatment or because of the absence of any treatment. If a disease is severe so that the mortality rate for sufferers from the disease is high and if no treatment is currently known it is clear that any substantial reduction in the mortality rate is a sufficient indication of the value of a new treatment. A more common state of affairs is the case of a well established treatment for a disease where two questions may arise: i. Is the customary treatment more effective than no treatment? it. Is the customary treatment less effective than one or more new possible treatments? 'Dept. of Mathematics, Brunel University, London. 455