XVII
PREDICTION BY COMBINED SCORES
Total scores based on group comparisons. Weighting critical sections. Total
scores based on the establishment of correlation. Multiple correlation coefficient
 and multiple regression equation. Other methods of combining
scores.
WHEN the predictive values of scores in the various separate
 tests or other measuring instruments have been determined,
 the next step is to find the predictive value of combined
 scores in all of these measuring instruments. Since
the valid tests will not all forecast vocational accomplishment
 with equal accuracy, they should be properly weighted
in obtaining a combined score. This combined score should
yield a more accurate prediction than the score in any one
particular test. Such a total score eliminates opinion based
only on general impressions of the combined significance of
test scores, thus retaining the principle of objectivity.
Here as in the preceding chapters the methods are treated
under two headings: those to be used where group comparisons
 have been made, and those to be used where correlations
 have been calculated.
TOTAL SCORES BASED ON GROUP COMPARISONS
Tests which have been evaluated by methods of group
comparison and for which critical sections have been determined
 should be combined with reference to critical sections
 and not to raw scores. To add raw scores would introduce
 an unintentional weighting unless correction were
made for differences in values of means and standard deviations
 (see pages 206 ff). It would introduce in this case an
additional error, inasmuch as critical sections are fixed with-20%