Letters Systematic Approach Sir: I read with interest your editorial on " T h e (Un)Scientific M e t h o d " in the October issue of ANALYTICAL C H E M I S T R Y . Systematic use of experi mental design is often a topic of dis cussion rather than a tool of wide ap plication. I would like to share my per sonal experience with the application of the systematic design. In evaluating a particular process problem at hand, we estimated the number of variables requiring scrutiny to be 12. If we had had to evaluate all 12 variables without prior screening (and reduction), the total number of experiments required would have been 2 1 2 or 4096. Systematic screening to identify the important variables re quired 20 experiments, and subse quent study of the selected variables increased the number by 2r where ρ is the number of important variables. If the number of important variables was three, then the total number of experiments required would be 31. For the particular process problem in eval
uation, a study involving 31 experi ments was considered too expensive and time-consuming. So we opted for a semi-systematic approach. We picked the three variables which in our opinion were the most impor tant, and made a systematic study of these three variables by conducting a total of 11 experiments. As it turned out, the three variables selected were not the important variables. However, while performing these 11 experi ments we suspected that two other variables could be more important. These two variables were then studied systematically (seven experiments). Success achieved at l a s t . . . the prob lem was identified and quickly solved. T h e systematic approach can also be applied in solving problems in ana lytical methods development. Applica tion here is generally much simpler since the experimental error is much smaller than the effect of the impor t a n t variables. Also, the variables are fewer (usually two or three), and the
time involved in using either system atic approach or trial and error meth od is small. T h e choice is often a mat ter of individual preference. I favor the systematic approach because it fa cilitates a better report of the results. Systematic approach is definitely the best approach in terms of scientif ic sense. In practice, however, it does not stand much chance when dealing with multivariable problems. In most cases, cost/benefit analysis goes against it, so we have to settle for a semi-systematic approach. In a few cases, we may not have a choice. T h e systematic approach may be the only way to optimize a process with multi ple variables which interact indepen dently as well as together. R. R a o G a d d e Analytical Research and Technical Services Westwood Pharmaceuticals Inc. 468 Dewitt Street Buffalo, N.Y. 14213
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26 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
