pg_0037
5.
Process Improvement
5.3.
Choosing an experimental design
5.3.1.
What are the objectives.
Planning an
experiment
begins with
carefully
considering
what the
objectives
(or goals)
are
The objectives for an experiment are best determined by a team
discussion. All of the objectives should be written down, even the
"unspoken" ones.
The group should discuss which objectives are the key ones, and which
ones are "nice but not really necessary". Prioritization of the objectives
helps you decide which direction to go with regard to the selection of
the factors, responses and the particular design. Sometimes prioritization
will force you to start over from scratch when you realize that the
experiment you decided to run does not meet one or more critical
objectives.
Types of
designs
Examples of goals were given earlier in
Section 5.1.2
, in which we
described four broad categories of experimental designs, with various
objectives for each. These were:
Comparative designs to:
choose between alternatives, with narrow scope, suitable
for an initial comparison (see
Section 5.3.3.1
)
r
choose between alternatives, with broad scope, suitable for
a confirmatory comparison (see Section
5.3.3.2
)
r
q
Screening designs to identify which factors/effects are important
when you have 2 - 4 factors and can perform a full factorial
(
Section 5.3.3.3
)
r
when you have more than 3 factors and want to begin with
as small a design as possible (Section
5.3.3.4
and
5.3.3.5
)
r
when you have some qualitative factors, or you have some
quantitative factors that are known to have a
non-monotonic effect (Section
3.3.3.10
)
r
Note that some authors prefer to restrict the term screening design
to the case where you are trying to extract the most important
factors from a large (say > 5) list of initial factors (usually a
fractional factorial design). We include the case with a smaller
q
5.3.1. What are the objectives.
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