Prerequisites for the concept of shop profit: Logic and memory

Several studies have shown that it is very difficult for children younger than 10 years old to understand the notion of shopkeepers' profits. In order to investigate the relationship between the development of this notion and the development of logico-arithmetical abilities and of STM span, 59 children (about 20 at each school grade from second through fourth) were administered the following tasks: (1) interview on buying and selling and shopkeepers' profits; (2) a task consisting of comparing expenses and income of a shopkeeper selling his goods at cost; (3) digit span forwards and backwards, and Case's (1985) counting span test. There was a strong association between the level of a child's conceptions of how shopkeepers make their living, and the procedures he or she adopted in comparing costs and income. Only low associations were found between the scores of children's performances on the memory tests and the levels they reached in the other two tasks.