Subtraction by indirect addition. A most efficient strategy for solving symbolic subtraction problems?

Children all over the world are traditionally taught to solve multi-digit subtractions of the type a - b = ?, such as 815 - 668 = ?, by subtracting the smaller number from the larger one. However, there is also another type of subtraction strategy, called subtraction by addition, whereby the child computes how much there is to be added to 668 to get at 815. Especially for subtractions with a very small difference between the two given numbers this latter strategy seems particularly suited. Remarkably, the subtraction by addition strategy is hardly studied and taught. We have shown in several studies that adults use it remarkably frequently, efficiently, and flexibly. But evidence on its use in children is very inconclusive. The major goal of the present research project is to demonstrate the computational superiority of this subtraction by addition strategy over the traditionally taught direct subtraction strategy in elementary school children below the age of 11-12, as well as children’s capacity to apply this efficient strategy in a flexible way, by means of a large-scale study that uses the choice/no-choice paradigm. Also, the links between children’s (efficient and flexible) use of this mental calculation strategy and their mastery of the underlying conceptual knowledge of subtraction, as well as its link with cognitive measures such as working memory load, are investigated.