The aim of the study was to analyze the development of children's adaptive expertise in computing sums and differences up to 100. We defined the adaptive nature of children's strategy choices on the basis of problem (addition, subtraction), achievement, and strategy performance (accuracy, speed). Sixty-nine 2nd graders of high, above-average, or below-average mathematical achievement level solved 4 types of additions and subtractions before and after regular classroom instruction in the topic. In both assessments, children answered the problems in a choice condition, in which they could choose between the jump strategy (45 + 28_(_); 45 + 20 = 65,65 + 8 = 73) and the split strategy (45 + 28 =_; 40 + 20 = 60, 5 + 8 = 13, 60 + 13 = 73) on each problem, and in two no-choice conditions, where they had to solve all problems with either the jump or the split strategy. Our results revealed differences in children's strategy choices on both assessments. The choices of high achievers indicated that they fit their strategies to problem and strategy performance characteristics. In contrast, above-average and below-average achievers' choices were unrelated to those characteristics. These results are interpreted in terms of the viability of different instructional approaches to enhance the acquisition of adaptive expertise, specifically, conceptual and investigative approaches.
