A Critical Comparison of Two Procedures for Antiperspirant Evaluation

9.74 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
The RM gives estimates of per cent reduction which have been modified
by the a priori mean ratio obtained at the time of the pretest measurements.
If this mean ratio were constant, it would not alter the per cent reduction
obtained relative to that of the SSEM. To the extent that this is not true, the
per cent reduction will be differenthan that given by the SSEM. Since it has
been shown that the ratios can be quite variable, it follows that the per cent
reduction estimates obtained may be substantially altered. This difference in
the estimates is obviously a function of the degree of variability of the pretest
ratios. Under these circumstances, the precision of the estimate becomes irrelevant
even though it is a function of the number of pretest determinations
as well as the factors operating in the SSEM. A questionablestimate is not
improved by reducing its variability.
It is possible, however, to utilize pretest data in a statistically valid manner
to adjust posttest results by the use of an analysis of covariance, and this
procedure might be shown to give more accurate, and precis estimates of p;'r
cent reduction than those given by the SSEM, if the pretest data are sufficiently
well-correlated with the posttest ratio. The mathematics of such an
analysis, however, differ from those of the ratio adjustment procedure, and it
is impossible to predict results without experimentation. When time permits,
we plan to investigate this matter. Meanwhile we recommend the SSEM over
the RM as the scientifically valid method, as well as the one of choice on the
bases of simplicity and low costr
ACKNOWLEDGMENTS
We wish to thank Dr. John Mandel for statistical advice xvhich has been helpful in
clarifying our thinking regarding a number of points. In addition, Mr. Giles Crane has
contributed statistical assistance, and Miss Linda Timberlake performed a large nmnber cf
statistical analyses in preparing the experiments. Finally, we would like to express our
gratitude to the Editor and reviewers of this Journal for their careful, complete, and very
comtructive comments.
(Received November 12, 1974)
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