A Systematic Approach to Planning for a Designed Industrial

If a nuisance factor is not measurable and thought to

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Unformatted text preview: he analysis. If a nuisance factor is not measurable and thought to be very influential, it may also be called an experimental risk factor. Such factors can inflate experimental error, making it more difficult to assess the significance of control variables. They can also bias the results. For the CNC-machining example, the nuisance factors are as shown in Table 5. Experiment designers have a set of passive strategies (randomization, blocking, analysis of covariante, stratified analysis) to reduce the impact of nuisance factors. These strategies can have a major effect on the experimental design. They may be constrained Table 4. Held-Constant Factor (units) 9 EXPERIMENT - Precision of lab tests unknown How to control (in experiment) Anticipated effects Use one type None Do runs after machine has reached 100 Use one “midlevel” operator Use one lot (or block on forging lot, only if necessary) None TECHNOMETRICS, None Slight FEBRUARY 1993, VOL. 35, NO. 1 DAVID E. COLEMAN 10 AND DOUGLAS Table 5. Nuisance Measurement precision-how known? Nuisance factor (units) Viscosity of cutting fluid Ambient temperature (OF.1 Spindle Vibration of machine during operation Standard viscosity 1- 2” F. by room thermometer (estimate) - ? by limits on the number of observations, costs of changing control-variable settings, and logistic considerations. In the CNC-machining example, the only nuisance factor to have potentially serious effects and for which blocking seems appropriate is the machine spindle effect (though it may be necessary to also block on titanium forgings). The machine has four spindles. requiring a design with four blocks or randomizing on all four. Blocking will introduce a bias in the estimates confounded with the blocking variable(s), whereas randomization will inflate the experimental error. The other two factors are dealt with by ensuring that they stay below levels at which problems may be encountered. 7. INTERACTIONS The interactions sheet is self-explanatory. Unfortun...
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