The spacing effect: Additions to the theoretical and empirical puzzles.

Four studies examined the MP-DP effect (spacing effect) in four quite different situations: recognition of letters, verbal discrimination, short free recall lists, and recall of MP items presented twice, with an intervening interval inserted to produce forgetting. MP-DP differences were found in all studies. Of particular interest were three interactions. Subjects with a low criterion of responding in the letter study lost the MP-DP effect over a 30-sec delay, and subjects with a high criterion did not. A clear MP-DP effect, but no lag effect, was found only with unmixed verbal discrimination lists. In free recall, a sharp lag effect was shown for words presented three times but not for words presented twice. A forgetting interval inserted between the two occurrences of an MP item did not appreciably aid its recall. The results were found to pose severe problems for current theoretical ideas about the spacing effect.