400 D. C. Cullum effectiveness was not reached until after at least five consecutive daily treatments. It was decided that three consecutive daily treatments, followed by a hot-room sitting on the third day, would be the best compromise between the need for repeated application of products and the need to do as many hot-room sittings as possible in a week, since it would permit sittings on Wednesday, Thursday and Friday in a normal working week. The procedure was thus essentially a modification of the 'SSEM' method of Wooding and Finkelstein. It is described in detail later. The chief purpose of this paper is first to define the basic requirements of a valid performance evaluation method and then to show that, in its modified form, our 3-day test satisfies these requirements. PRINCIPLES THE PURPOSE OF THE TEST It ought to be self-evident that the first thing to do in devising any kind of test is to define precisely the information the test is required to produce. In product development, performance tests normally serve one of three purposes: to provide support for advertis- ing claims to assess the extent to which users will be able to perceive that the product works and to tell product development scientists whether they are moving in the right direction. In practice, 'the right direction' will usually be defined in terms of either advertising support or user perception, so there are really only two purposes. It is unlikely, though not impossible, that one fixed procedure will serve both of these. The hot-room stimulus and the methods used for assessing sweat output are artificial, and the only purpose such a test can serve is to rank products in order of efficacy under an arbitrary set of conditions. For reasons to be discussed later this will not necessarily be the order of efficacy perceived by users. MODE OF APPLICATION OF PRODUCTS In recommending a test procedure we may prefer on the one hand complete control over all the variables, which does not resemble what happens in real life, or on the other hand complete realism, which will entail considerable variation in modes of application of the product, applied stimuli, and parameters and criteria of efficacy. Usually we shall strike some kind of compromise between these extremes. The question of how best to apply the products is a particularly difficult one, yet it is surprising how little attention it has received in the published literature. Its supreme importance lies in the fact that when two different products are compared, either In separate tests against a common control or directly in one test, the result in the sense of a decision as to which is better may be dictated by the amounts applied. Although the hot- room test is artificial and can achieve no more than a ranking of products, it is surely implicit that we are interested in the ranking order under conditions of actual use. The problem is most easily seen when a comparison is made between products of different types, for example a roll-on and an aerosol. It we apply eight strokes of the roll-on and a three-second spray with the aerosol, perhaps the aerosol will appear better. If we give twelve strokes and a two-second spray, perhaps the roll-on will appear better. Indeed, such a comparison has no discoverable meaning unless the products are applied in a manner which closely resembles the mode of application in real use.

Rapid hot-room testing of antiperspirants 401 An obvious solution to the problem is to find'out by experiment the mean weight•of each product used by a large assembly of subjects, and to apply that weight to each experimental axilla. This is not very practicable, since it would have to be done for every product tested. It is also scientifically wrong. The amount of roll-on used in real life is markedly influenced by the topography of the user's axillae, and this can vary very considerably. The mean amount can be grossly excessive for subjects with small, deep axillae, and not nearly enough for subjects with broad, flat axillae. The same is probably true, perhaps to a lesser extent, for aerosols. The application of the mean amount to all axillae is therefore no more realistic than the application of any other constant amount. Another difficulty is that although the ratio of the mean amounts of the two products may be known, it is not justifiable to assume that all users would apply them in that ratio one subject might use a lot of roll-on and a little aerosol while another might do the opposite. The same considerations apply even when the two products are of the same type. Aerosols differ in their non-volatile content, discharge rate, coldness and other charac- teristics, and real users do not use a constant spray time nor a constant discharged weight. The use of either of these in a test method carries the risk of giving an unfair advantage to one or other of the products. The most realistic procedure would be to allow the subjects to apply the products themselves. This carries the obvious risk of increased scatter in the results and a con- sequent need for larger numbers of subjects. On the other hand, any other procedure carries the risk of generating ranking orders different from those experienced in actual use. The method to be described employs a two-second spray applied by a member of the laboratory staff, when aerosol products are being tested. This is justified by the admittedly tenuous arguments that it is what most other laboratories do and it is what the manu- facturers of most aerosol products recommend on the pack. Also from our own crude estimates it seems to be a reasonable approximation to the normal practice of a majority of users. For the infrequent experiments in which products of different types are com- pared, we have adopted the practice of allowing the subjects to apply the products themselves. So far we have too little experience to be able to weigh the advantages and disadvantages of doing so. What must always be borne in mind is that the result found in an individual test is not necessarily the same as that which would have been found had the test been conducted under different conditions. When we say, 'Product A was x•o better than product B', we always imply, 'under the particular conditions of our test'. CHOICE OF PARAMETER TO BE MEASURED The primary measurement is, of course, the number of milligrams of sweat secreted by an axilla during a specified collection period. The parameter of efficacy which this is used to assess may, however, be one of several things. In making a choice it is essential to bear in mind the function of a control group or substrate in any experiment: the function of the control substrate is to provide the best possible estimate of how the test substrate would have responded if it had received the control treatment instead of the test treat- ment. It follows that the two substrates (groups) must be alike with respect to the para- meter to be measured. This means that they must respond in the same way when they both receive no treatment and when they both receive the same treatment. The parameter