Page 27 - Applied statistics and probability for engineers

Section 1-2/Collecting Engineering Data 5

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
examples of these data-collection methods.

1-2.2 RETROSPECTIVE STUDY
Montgomery, Peck, and Vining (2012) describe an acetone-butyl alcohol distillation column for
which concentration of acetone in the distillate (the output product stream) is an important variable.
Factors that may affect the distillate are the reboil temperature, the condensate temperature, and the
relux rate. Production personnel obtain and archive the following records:
r The concentration of acetone in an hourly test sample of output product
r The reboil temperature log, which is a record of the reboil temperature over time
r The condenser temperature controller log
r The nominal relux rate each hour
The relux rate should be held constant for this process. Consequently, production personnel
change this very infrequently.
Hazards of Using A retrospective study would use either all or a sample of the historical process data archived
Historical Data over some period of time. The study objective might be to discover the relationships among the
two temperatures and the relux 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 relux rate and acetone concentration
because the relux rate did not change much over the historical period.
2. The archived data on the two temperatures (which are recorded almost continuously) do
not correspond perfectly to the acetone concentration measurements (which are made
hourly). It may not be obvious how to construct an approximate correspondence.
3. Production maintains the two temperatures as closely as possible to desired targets or set
points. Because the temperatures change so little, it may be dificult to assess their real
impact on acetone concentration.
4. In the narrow ranges within which they do vary, the condensate temperature tends to
increase with the reboil temperature. Consequently, the effects of these two process vari-
ables on acetone concentration may be dificult to separate.
As you can see, a retrospective study may involve a signiicant amount of data, but those data
may contain relatively little useful information about the problem. Furthermore, some of the
relevant data may be missing, there may be transcription or recording errors resulting in outli-
ers (or unusual values), or data on other important factors may not have been collected and
archived. In the distillation column, for example, the speciic concentrations of butyl alcohol
and acetone in the input feed stream are very important factors, but they are not archived
because the concentrations are too hard to obtain on a routine basis. As a result of these types
of issues, statistical analysis of historical data sometimes identiies interesting phenomena, but
solid and reliable explanations of these phenomena are often dificult to obtain.

1-2.3 OBSERVATIONAL STUDY

In an observational study, the engineer observes the process or population, disturbing it as
little as possible, and records the quantities of interest. Because these studies are usually
conducted for a relatively short time period, sometimes variables that are not routinely
measured can be included. In the distillation column, the engineer would design a form to
record the two temperatures and the relux rate when acetone concentration measurements
are made. It may even be possible to measure the input feed stream concentrations so that
the impact of this factor could be studied.