Ss were required to count the number of occurrences of each of several different symbols (categories) presented sequentially as a function of the number of categories (2, 3, or 4), the total number of items (8, 12, 16, or 20) and the rate of presentation (1 item every 0.6, 1.0, 2.0, 3.0, and 4.0 sec.). The greater the number of categories, the greater the total number of items, or the faster the rate of presentation, the poorer the performance. Of primary importance, however, is the complex interaction between these variables. The results are explained primarily on the basis that both presentation rate and number of categories affect the time available for rehearsal of the information while trial length results in an accumulation of errors with time. (PsycINFO Database Record (c) 2006 APA, all rights reserved)