Standard sentences, prose passages, and poems have been used to control input in studies of retention; but discrete verbal units (such as words or sets of letters) are most frequently employed. The letters usually comprise lists of consonant syllables (three consonants; e.g., RQK) or so-called nonsense syllables (consonant-vowel-consonant; e.g., ROK). The order in which verbal units are to be learned and to be recited may be left to the subject (free recall). A schoolchild who can recite the names of all African countries probably has learned such a free-recall task. Units also can be presented serially (in a constant order), the subject being asked to recite them in that order; reciting the alphabet in the usual way represents such serial learning.
Pairs of words may be offered; in such paired-associate tasks the subject eventually is asked to produce the missing member of each pair when only one word is shown. This is akin to learning English equivalents for words from another language.
For these and similar tasks investigators commonly permit subjects enough practice trials to reach some preselected criterion or level of performance. This level effectively defines an immediate retention score against which later forgetting may be measured. Subsequent tests of retention are then made to investigate the rate at which forgetting proceeds. This rate tends to vary with the methods used, basically those of recall, recognition, or relearning.
Other ways to improve memory. A good way to ensure remembering a piece of information is to study it long after you think you know it perfectly. This process is called overlearning. The more thoroughly you learn something, the more lasting the memory will be.
Subjects may be asked to reproduce (recall) previously learned data in any order or in the original order in which they were learned.
In a free-recall test the instructions might be: 'Yesterday you learned a list of words; please write as many of those words as you possibly can as they occur to you.' For the paired-associate task the subject may be told: 'Yesterday you learned some pairs of words; I will show you one word from each pair and you try to give the other.' He may be paced, being limited to a few seconds to produce each word; or he may be unpaced, being given no rigidly specified limits.
If retention of any kind is to be measured over different periods (e.g., an hour, a day, a week) a separate group of individuals should be used for each period. The reason is that the very act of remembering constitutes practice that keeps memory lively, tending to give misleading underestimates of the rate of forgetting if the same subjects are tested over successive intervals.
Psychologists have studied the course of forgetting over time extensively. Most often, rapid forgetting occurs at first, followed by a slower rate of loss. Improvement in the amount of material retained, however, can be achieved by practising active recall during learning, periodically reviewing the material, and over learning–that is, relearning the material beyond the point of bare mastery. This process relates memory ability to comprehension.
Four traditional explanations of forgetting have been provided. One is that memory traces fade naturally over time as a result of organic processes occurring in the brain, although little evidence for this notion exists. A second is that memories become systematically distorted or modified over time. A third is that new learning often interferes with or replaces old learning. Finally, some forgetting may be motivated by emotional needs and wishes–as when unpleasant childhood experiences are repressed or denied. These can be 're-memorized' under hypnosis or in psychotherapy