Thirty-three novices and 33 expert calculator users took a test in which they predicted what number would be in the calculator's display after a sequence of button presses (such as 2 + 3 +). Subjects' answers did not depend on the brand of calculator they actually owned. Simple production systems models were fit to the performance of each subject. Subjects differed greatly with respect to when they thought an expression would be evaluated, the order in which a chain of calculations was evaluated, and whether the display would be incremented. (Author)