STATISTICAL EVALUATION OF CUTANEOUS IRRITANTS 821 2.00 -- • Lgo • •o .-- 140 ._ = 120 "Pressure" Patch / B I "Regular" Patch o/ At D, (Zhrs. Obs. Time) 2.00 180 160 1.40 120 o B2 •..' Pre$sure" Patch/ ß I "•egu or" Pa c At Dg (18hrs. Obs. Time) Loo I I Ioo I A• A 2 A• (• O/o) (4%) (• O/o) Irritant Conc., % Figure ,5. Triple interaction (ABD). Concentration X type of patch X ohs. time I (4%) Irritant Conc., % greater irritation effect with the pressure patch, the difference, if any, between pressure and regular patches is the same at both concentrations. DESIGN OF SECOND EXPERIMENT The second design was worked out and run after noting and dis- cussing the findings just given. Several important differences were to be built into this new experiment. From the beginning, it had been felt desirable to run an experiment in which concentration was used at more than two levels, so that the nature of the concentration response (e.g., curved or linear) could be studied. Such a procedure carries additional advantages in that it is often possible to obtain greater sensitivity in the analysis of the data. Additional levels of concentration were usable only if one or more of the original four variables was eliminated and no new ones added, or if the new experiment was made larger. For practical reasons, the latter was undesirable. Since contact time had been found not to interact with any of the other variables in the first experiment, it was possible to eliminate this factor from the second one, because all neces- sary information about its main effect response was already available.

822 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS This left three variables for the new experiment, with contact time held constant at some fixed level (preferably close to one of those used in the first experiment). It was decided to use 24 hours, because the longer time gave higher irritation values compared to the shorter period, and thus took greater advantage of the range of the scale used. The three remaining variables to be used in this experiment, and the levels decided upon were: (A ) Concentration of irritant (Duponol C) Ai = 0.5% A• = 1% A• = 2% A4 -- 4(7/0 (B) Type of Patch B• = Regular B.. = Pressure ( U) Observation Time C• = 2 hours Ca = 24 hours The spacing of the levels of concentration (each double the previous one) was adopted because: (a) it was believed that, like many chemical concentration and biological dosage effects, the increase in response might well become progressively smaller as concentration increased (this was also indicated by some of the preliminary work), and (b) this type of spacing made the data amenable to a particularly simple type of statistical analysis for the curvature effects. The same system of blocking and replication as before was used i.e., three repetitions of two blocks (two subjects) each. None of the subjects were the same as those used before. The basic experimental design was that known as a "split plot," in which a main effect instead of an interaction is confounded between ("mixed with") blocks. Thus, it will be noted (Fig. 6) that only one of the two types of patch was used on a given subject in each pak. This procedure results in loss of sensitivity for the detection of the con- founded effect (but not complete loss of information, since there was replication). It was decided to use the patch type variable, B, for the splitting, because the previous experiment indicated that this main effect was of little value in interpretation of the other factors, and it had already been established as a strongly significant variable, so that there was little doubt of its reality and little need for further verification.