Basically, ACT-R's production strength mechanism is the equivalent of Thorndike's Law of Exercise while its rules for learning probability and cost become the equivalent of Thorndike's Law of Effect (Thorndike, 1913). The research of Lovett (Lovett & Anderson, 1996) nicely illustrates the separation of these two factors. She has explored the effect of practice with problem-solving operators that yield differential success. Figure 4.4 shows her task which she called the building-sticks task. It is an isomorph of Luchins' waterjug problem (Luchins, 1942; Luchins & Luchins, 1959). The goal in this task is to construct a stick of a particular desired length given three stick lengths from which to build (there is an unlimited supply of sticks of each length). There are basically two strategies to use in trying to solve this problem. The undershoot strategy is to take smaller sticks and build up to the target stick. The overshoot strategy is to take the stick longer than the goal and cut off pieces equal in length to the smaller stick until one reaches the target length. Lovett arranged it so that only one strategy would work for a particular problem and gave subjects problems where one of the two strategies worked on a majority of the problems (and she counterbalanced over subjects which was the more successful strategy).