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"F-value specified to enter or delete variable, <4.0. F-value is univariable F-value.

"F-value specified to enter or delete variable, <4.0. F-value is univariable F-value.

good term, once selected, may be later eliminated (Table 6) because in combination with the set of terms then at play, the term doing the replacing steps up the discriminating power more than the term selected earlier did.

Table 8 shows for SRA the same sort of thing as described above. At the fourth step, the computer had to go over possible combinations again and again to find the combination that yielded the highest coefficient of determination. In this instance the gain in effectiveness of prediction is so slight that most investigators would decide to go with a three- or four-term equation rather than one having a larger number of terms.

In some cases, investigators may not select the most efficient LDA or MRA equation because while there is one best equation there usually is a whole tier of second-best equations. A user of LDA or MRA may decide that one of the tests among the combination yielding the most efficient equation is too lengthy or too expensive to run, that a slight sacrifice in efficiency is merited in exchange for a more speedy or less costly test. In one example, an attempt was made to determine which compounds in oil of onion were the essential ones to an oil of onion flavor so that a synthetic product could be formulated (24). The most effective equation generated did not include compounds that the literature and the experience of the investigators indicated were compounds likely to be essential to onion flavor. The ones not selected were colinearly related (see the discussion above concerning correlated versus orthogonal terms) to the ones selected; thus both would not be included in the same equation. The researchers, therefore, forced into the equation the gas chromatographic peaks they thought were critical to authenticity of onion flavor. This goes back to the statement made in the opening paragraph that a chemical test may be precise but miss the target because it is not measuring a key sensory quality. Computer programs are blind. They will select the most efficient equation. Sensory technologists and chemists sometimes must

Table 8. R2 Improvement and F- Value Decline as Variables are Added, Stepwise Regression

Step

Variable added

Variable deleted

R2

Regression F-value

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