When a subject (S) was asked to list all the items belonging to a specified category, cumulative totals of items produced exhibit a smooth curve as a function of time. The equation proposed by Bousfield and Sedgewick in 1944 was fitted to single sequences of individual 5s, and it was suggested that either the S scans at a constant rate and scans exhaustively or that the density of relevant items under scanning decreases linearly. When the retrieval was repeated in immediate succession, the functional form of the sequences changed and two models were proposed: one assuming operation of two parallel scanning channels and the other assuming the change of retrieving rate to an exponential form.