Investigators of schizophrenic cognition often produce 2 or more tasks
of differing difficulty levels by manipulating a variable that affect
s the accuracy of both normal and schizophrenic individuals; the inves
tigators find that the variable also affects the difference between th
e groups in accuracy and conclude that the variable taps a schizophren
ic differential deficit. An alternative hypothesis is that task differ
ences in true-score variance artifactually produce the finding. For fr
ee-response tasks, group differences tend to be larger when difficulty
is near 50%. The authors illustrate a new method of controlling this
artifact by selecting items for hard and easy tasks on opposite sides
of 50% difficulty and equidistant from it. Using this design with an a
nagram task, they found that schizophrenic and normal individuals diff
er no more on hard anagrams than on easy ones, and they propose the de
sign for testing hypotheses concerning schizophrenic deficit on tasks
that differ in difficulty.