Page 19 - Applied Statistics And Probability For Engineers

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1-2 COLLECTING ENGINEERING DATA 5

The wafers-from-lots example is called an enumerative study. A sample is used to make
an inference to the population from which the sample is selected. The connector example is
called an analytic study. A sample is used to make an inference to a conceptual (future)
population. The statistical analyses are usually the same in both cases, but an analytic study
clearly requires an assumption of stability. See Fig. 1-5, on page 4.

1-2 COLLECTING ENGINEERING DATA

1-2.1 Basic Principles

In the previous section, we illustrated some simple methods for summarizing data. In the en-
gineering environment, the data is almost always a sample that has been selected from some
population. Three basic methods of collecting data are
A retrospective study using historical data
An observational study
A designed experiment
An effective data collection procedure can greatly simplify the analysis and lead to improved
understanding of the population or process that is being studied. We now consider some ex-
amples of these data collection methods.

1-2.2 Retrospective Study

Montgomery, Peck, and Vining (2001) describe an acetone-butyl alcohol distillation
column for which concentration of acetone in the distillate or output product stream is an
important variable. Factors that may affect the distillate are the reboil temperature, the con-
densate temperature, and the reflux rate. Production personnel obtain and archive the
following records:

The concentration of acetone in an hourly test sample of output product
The reboil temperature log, which is a plot of the reboil temperature over time
The condenser temperature controller log
The nominal reflux rate each hour
The reflux rate should be held constant for this process. Consequently, production personnel
change this very infrequently.
A retrospective study would use either all or a sample of the historical process data
archived over some period of time. The study objective might be to discover the relationships
among the two temperatures and the reflux rate on the acetone concentration in the output
product stream. However, this type of study presents some problems:
1. We may not be able to see the relationship between the reflux rate and acetone con-
centration, because the reflux rate didn’t change much over the historical period.
2. The archived data on the two temperatures (which are recorded almost continu-
ously) do not correspond perfectly to the acetone concentration measurements
(which are made hourly). It may not be obvious how to construct an approximate
correspondence.