Design of Experiments and Statistical Process Control - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 228

10%
20%
30%
40%
50%

60

50

40

30

20

10

1 2 3 4 5 6 7 8

Time (hours)

Figure 13.5 Contour Chart of Yield versus Temperature and Time

Temperature Yield

10 ° 10%

20 ° 22%

30 ° 19%

40 ° 8%

50 2%

°

Figure 13.6 Yield versus Temperature

run under the same conditions gives yields between 80% and 90%. An impa-
tient experimentalist might check the importance of agitation and perform
two experiments, one at low agitation, obtaining a yield of 90%, and a second
experiment at high agitation, obtaining a yield of 80%. It would be logical to
think that agitation is important and that it is better to use low agitation. How-
ever, in this example, agitation has no effect and the impatient experimentalist
was just observing random variation.
Another potential pitfall is confounding factors. Imagine a reaction where
time is very important and temperature does not have much effect. A set
of experiments to determine the effect of temperature might give increasing
yields as temperature is increased, but the reason might be that experiments
take longer to run at higher temperatures due to the time to heat the reaction