ONE-TRIAL LEARNING WITH CONTROL OF ITEM DIFFICULTY,
WASHINGTON UNIV SEATTLE PSYCHOPHYSICS LAB
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Ratings of difficulty were obtained for each of 2500 letter-number pairs. The ratings were used to construct three pools of items Homogeneous Easy, Homogeneous Difficult, and Heterogeneous. Rocks 1957 experiment was repeated thrice, once with each pool of rated items. Each replication used 30 Ss. The manipulation of rated item-difficulty permitted a direct attack on the widely discussed problem of item-selection in Rocks procedure. Notwithstanding the control of item-selection, the performance of Ss in the drop-out condition did not differ significantly from the performance of Ss that learned in the ordinary way, with repetition. The geometric distribution provided a good fit to the obtained distributions of waiting times to the first correct response to an item. This finding accords nicely with an all-or-none view on associative learning.