Mental set and mental arithmetic

State University of New York, Buffalo, New York 14226 Ss performed mental arithmetic problems in which they added, subtracted, or multiplied two one-digit numbers. The presentation order of the operator symbol and the digits was varied. With three possible operators, presentation of the operator prior to the digits (OD) led to faster RTs. With two possible operators, the opposite order (digits prior to operator, DO) led to faster RTs, because RTs in the OD condition were unaffected by the number of possible operators. These results are discussed in terms of the trade off between accessing active memory for a small number of items in the DO condition vs retrieving information from relatively large tables in long-term memory in the OD condition.