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Unformatted text preview: olicited in item 8 of the guide sheet.
Predicted effects for the response variables may
be available from the knowledge sources previously
experts, and experiments. Quantitative predicted effects are preferable, but experimenters may not be able to provide more than qualitative indications. Even if uncertain, the exercise of
attempting to predict the outcome of the experiment
before it is run can foster good interaction within the
team and often leads to revised
choices of settings. An additional advantage is that
the predictions will always be wrong, so it is easier
to see what knowledge has been gained through experimentation. PLANNING 5. HELD-CONSTANT FOR A DESIGNED FACTORS Held-constant factors are controllable factors whose
effects are not of interest in this experiment. Most
experimenters think in these terms: “For this experiment, I want to study the effect of factors A, B, and
C on responses y, and y,, but all other control variables should be held at their nominal settings, and I
do not want extraneous factors distorting the results.” This sheet was developed to ensure the “all
other control variables held at their nominal settings”
condition. (The next sheet is used to help ensure that
“there are no extraneous factors distorting the results.“) For the CNC-machining example, the heldconstant factors are as shown in Table 4.
The sheet in Table 4 can force helpful discussion
about which factors are adequately controlled and
which factors are not. In so doing, it is often necessary to consult experts to help prioritize factors,
recommend preexperiment studies to assess control,
or develop control strategy.
For example, in the CNC-machining case, this sheet
resulted in the recognition of the fact that the machine had to be fully warmed up before cutting any
blade forgings. The actual procedure used was to
mount the forged blanks on the machine spindles
and run a 30-minute cycle without the cutting tool
engaged. This would allow all machine parts and the
lubricant to reach normal, steady-state operating
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This document was uploaded on 02/12/2014.
- Fall '14
- The Land