lag effect
a cognitive phenomenon in which long-term retention of to-be-learned information improves with increased separation (lag) between repeated presentations of the information within a single period of study. Such lag is obtained by interspersing the material of interest with additional material or by increasing the number of overall elements between item repetitions. For example, a student preparing for a Spanish vocabulary exam would remember more from a study session by reviewing one long list of words multiple times instead of dividing it into three smaller lists because there would be a longer lag between the repeated presentations of each word in the former method. Although some use the terms synonymously, the spacing effect is an instance of the lag effect, referring to the improvement in memory that occurs when repetitions of material are distributed over multiple study sessions rather than when the material is studied in a single session (massed practice). One explanation of the lag effect is the differential-organization theory, which proposes that people organize presented material into overlapping groups (e.g., of interassociated words); that as lag increases, the groups overlap less and less; and that the probability of recalling a given word thus increases as a function of the number of different groups of words with which it has been associated. See also primacy effect; recency effect. [identified in 1967 by Arthur W. Melton]