| 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.
Examples of goals were given earlier in Section
5.1.2, where 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)
-
choose between alternatives, with broad scope, suitable for a confirmatory
comparison (see Section 5.3.3.2)
-
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)
-
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)
-
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)
-
Response Surface modeling to achieve one or more of the following
objectives:
-
hit a target
-
maximize or minimize a response
-
reduce variation by locating a region where the process is easier to manage
-
make a process robust (note: this objective may often be accomplished with
screening designs rather than with response surface designs - see Section
5.5.6)
-
Regression modeling
-
to estimate a precise model quantifying the dependence of response variable(s)
on process inputs.
After identifying the objective listed above that corresponds most closely
to your specific goal, you can
-
proceed to the next section where we discuss selecting
experimental factors
and then
-
select the appropriate design named in section 5.3.3
that suits your objective (and follow the related links).
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