442 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS product and later use the test, whilst the other panel would use the two products in reverse order. If, however, the test product has any real effect, it is vital to know how soon the dandruff level reverts to normal when treatment is discontinued, otherwise a "carry-over" effect will be operative during the control phase for the panel starting with the test product, results during the control phase will also depend on the actual time required to regenerate dandruff by infection or other means. The post-treatment zone illustrated in Fig. 2 shows that several weeks are required to re- establish the original pre-treatment dandruff level when an active anti- dandruff agent has been used. A practical solution might be to allow a prolonged time-lag between test and control phases, but this is likely to prolong the whole procedure of experiment to an unacceptable extent. A "cross-over" trial ignoring the points noted would tend to underestimate the efficacy of a treatment. o i i , 5 10 I PIE-TREATMENT IN WEEKS I I TREATMENT I I_.. {TESI Figure 7 Results of 16 weeks' treatment with shampoo containing 2 % w/w zinc bis(pyridine-2-thiol l-oxide) on a case of moderate-severe dandruff. (Detailed method of inspection). It is almost a truism to suggest that the statistician who will have to determine the significance of the results, should be fully consulted at the planning stage of a clinical trial. It is the authors' view that a carefully- planned and well-conducted trial on a limited panel is far more valuable than a poorly-conceived investigation carried out on a relatively vast scale an essential feature of sound planning is to ensure that the results will be adequate and in a suitable form for statistical analysis.

THE CLINICAL EVALUATION OF ANTIDANDRUFF SHAMPOOS 443 SPECIAL PROBLEMS OF DANDRUFF TRIALS One of the main considerations is the decision on what is to constitute a favourable result for an individual participant. It may, for example, be shown that any shampoo, whether medicated or not, will remove some 70% of ('•andruff scale during the actual process of shampooing. In the absence of effective treatment, however, dandruff will return to approximately the original level within about five days what matters to the subject is that this recurrence should not happen, at least to the same extent. Hence an antidandruff shampoo can only be classed as effective if it significantly and consistently reduces the dandruff levels attained five days after shampooing and it is appropriate always to take measurements at this timing as far as possible. It should not be forgotten that clinical trial techniques stand or fall by the extent of cooperation achieved with the participants, and there is an ever-present hazard that some of the subjects may not really have used the test or control products at all, especially if they are unpleasant or complicated in use or deemed to be ineffective. Similarly precise timing of observations after shampooing is a target to be aimed at and it has to be hoped that minor discrepancies will cancel out between the panels. Recruitment of subjects for dandruff studies presents surprising difficulty. Obviously someone who has no dandruff cannot show any beneficial effect, however efficacious the treatment. It is therefore necessary to recruit at least moderately severe sufferers, though it is also desirable to have some subjects who are only slightly affected in case either the test or control product demonstrates an adverse effect. It is quite impossible to ensure that a sample of subjects in a thai is fully representative of the population as a whole, if panels are to be kept within manageable propor- tions for instance, the source of infection in one geographical area may differ from others and may also differ in response to particular treatments (assuming that dandruff results from infection). If the subjects for a trial are recruited from an essentially "closed" community such as a boarding school, the risk of bias due to atypical sampling may be serious. On the other hand, it may be much easier to obtain the desired level of cooperation in such a community and far simpler to make suitable arrangements for conducting inspections. Probably the most satisfactory answer in practice is to utilize several such "closed" communities for the full evaluation of a new product. Subjects who are skin patients attending hospital are not ideal for the purpose, as they may not respond in the same way as "normals". Some published clinical trials on dandruff appear to have been restricted