STATISTICAL EVALUATION OF CUTANEOUS IRRITANTS 815 d 8 d, IL• in square crossed Lzm adheg,ve tape str,ps with guaze'• ker chip I In Iquore Saran f \ • 2 ,n x • m gauze pads .,,,,,,•,•r,,,,,,.,,,/,,,x,•,•,,•,, •in. lquare bloHing poper Figure $. "Pressure" patch able to make future comparisons with other small groups as a measure of the variability among separate experiments. The preliminary work had already suggested that such variability would be small once the subject-to-subject effect was removed. Isolation of subject differences called for the use of "blocking" and "confounding," which are widely used in statistical designs in bioassay and in chemical applications research. Essentially, blocking is a technique whereby, in the present case, the influence of subject-to- subject differences on the results is isolated, so that, if successful, it has little or no effect upon the differences of interest represented by such factors as irritant concentration, contact time, etc. Confounding is a technique of "mixing" effects of minor interest with blocks or subjects in order to increase the precision of measurement of more important variables. If it is assumed before an experiment that intersubject differences are substantially greater than intrasubject variation, smaller values of experimental error can be expected when these techniques are used.* The remaining intrasubject error, once this major source is eliminated is that likely to result from relatively small differences in location on the skin, patch application techniques, estimation of scores, etc. It is not intended that this presentation provide a complete "cook- book" description of the experimental design and statistical analyses used these may be obtained from several sources listed in the bibliog- raphy (7-15). Therefore, the experimental design, final analysis of variance tables, and conclusions are given, but the statistical calcula- tions are omitted This experiment was to include the above four variables, each to be tested at two levels in a four variable factorial design. The whole * The sensitivity of the experimental design and analysis in testing the various factors for significance depends: (a) upon the magnitude of the apparent effect due to changing a factor from one level to the next, and (b) upon the magnitude of the experimental error. The former is controllable only by the spacing set between two levels of the factor and upon its inherent ability to cause variation, but the latter depends upon the design characteristics.

816 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS design was to be repeated (replicated) three times, with two blocks or subjects in each replicate, confounding the four-factor interaction between each pair of blocks. The levels of the four variables were: A (Concentration of Irritant) A• = 1% Duponol C A•. = 4% Duponol C B (Type of Patch) B• = Regular patch (ef. Fig. 2) B•. = "Pressure" patch (cf. Fig. 3) C (Patch Contact Time) C• = 6 hours C•. = 24 hours D (Observation Time) D• = 2 hours after removing patch Ds = 18 hours after removing patch The factorial combination of the above resulted in 16 "runs" to be repeated in each of the three replicates. Each replicate consisted of two "blocks" or subjects, with eight runs applied to each. The quad- ruple interaction, ABCD, was confounded between each pair of blocks i.e., its effect, which was assumed a priori to be of no intrinsic interest, was "mixed" with that of the subjects. In the diagram which follows, standard two-level factorial notation is used. Thus, one of the sixteen runs was A•B•C•D• (1% Duponol, regular patch, 6 hours contact time and 2 hours' observation time). Another, for example, was A•.B2CiD• (4% Duponol, pressure patch, 6 hours, 2 hours). These two runs are designated in standard notation as run (1) and run ab, respectively. This notation includes the appro- priate lower case letter if the factor referred to is used at the high level, and omits this letter if the factor is used at its low level. Thus, the 16 runs in each replicate were: 1 (A•B•C•D•) a (A2B 1CiD,) b (A1B2C,D•) ab (A•B•C, D1) c (A•B•C•.D1) ac (A2BiC=D•) bc (A•B2C=D•) abc (A=B=C=D•) d (AiB•C•D•) ad (A2B•C•D•) bd (A•B2C•D2) abd (A2B2C•D•.) cd (A•B•C•D•.) acd (A•.B •C2D•.) bcd (A•B2C•.D•.) abcd (A2B•.C•.D•.)