Remembering a list of two-digit numbers

The way subjects remember a list of two-digit numbers has been examined in some detail. It is found that intrusions in free recall are not random. They resemble omissions in having the same first digit but not in other ways. This non-randomness of recall errors has been used to construct recognition tests of varying difficulty. Numbers which occurred commonly as intrusions were difficult to distinguish from the correct items when used as distractors in recognition tests. The experiments suggest that the previously observed relationship between recognition efficiency and number of alternatives (Davis, Sutherland and Judd, 1961) can be attributed to the increased probability that such intrusions will be included when the total number of distractors is increased.