"Seventy-two rats were trained to run down an alley for food. Half were trained with a 15-sec. interval between trials and half with a 15-min. interval. Each training group was divided for extinction, half being extinguished with the 15-sec. interval and half with the 15-min. interval. The measure of strength of response was time to traverse the alley . . … Extinction proceeded at a significantly faster rate when extinction trials were spaced than when they were massed . . … For the group trained with massed trials, spaced extinction was very significantly faster; but for the group trained with spaced trials, the difference, while in the same direction, was not significant." (PsycINFO Database Record (c) 2006 APA, all rights reserved)