Mental set and mental arithmetic*

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 tradeoff 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. Rational analysis of complex mental activity often leads to the positing of internal structures that are said to be hierarchically organized in the sense that the processing of specific information (or signals, exemplars, or data) is determined by the activation of a rule (o