XVII 
PREDICTION BY COMBINED SCORES 
Total scores based on group comparisons. Weighting critical sections. Total 
scores based on the establishment of correlation. Multiple correlation co- 
efficient and multiple regression equation. Other methods of combining 
scores. 
WHEN the predictive values of scores in the various sepa- 
rate tests or other measuring instruments have been de- 
termined, the next step is to find the predictive value of com- 
bined scores in all of these measuring instruments. Since 
the valid tests will not all forecast vocational accomplish- 
ment 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 com- 
parisons have been made, and those to be used where cor- 
relations 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 de- 
termined should be combined with reference to critical sec- 
tions and not to raw scores. To add raw scores would in- 
troduce an unintentional weighting unless correction were 
made for differences in values of means and standard devi- 
ations (see pages 206 ff). It would introduce in this case an 
additional error, inasmuch as critical sections are fixed with- 
20%