PREDICTION BY COMBINED SCORES
2. Proportion of all failures who score within the critical section;

3. Difference in proportions;
4. Standard error of the difference in proportions, using formula
 10 and considering any score within a critical section
 as the equivalent of a certain answer to a question;
5. Difference in proportions divided by its standard error.
If a person scores within a critical section, he is credited
with the amount of this fifth value in place of his raw score.
His total score is the sum of such values which he earns.
Weights may be assigned to critical sections in accordance
with the probability that an individual will achieve success
 if he scores within the critical section. Thus if 90% of
the individuals in the critical section are successes, the
probability of success of any person who scores within that
section is 9o/100, provided the subjects of the investigation
are representative. Similarly, if 20% of the individuals in
a critical section are successes, the probability of success of
any individual who scores within that section is 20/100.
The weights assigned to the critical sections may be in accordance
 with this ratio or percentage.
Note that critical scores are not determined on this basis,
but on the basis of difference in percentages of each total
group (successes and failures) found within the critical
section. If 20% of the individuals in a critical section were
successes, and at the same time 20% of all individuals tested
were successes, no significance would attach to the critical
section. This would not, strictly speaking, be a critical
section. But if 20% of the successes and 70% of the failures
 scored within the critical section, it would be very
significant.
Weights may be assigned to the critical sections in accordance
 with the reliability of the test or critical section;
that is, in accordance with the length of the test, its continued
 significance with different groups, or the relative
number of cases in the critical section. The latter is a practical
 index of the reliability of the critical section. A criti-208