Mathematical thinking skills were measured with nine commonitems. Five of these were from the Mental Arithmetic task, an adaptation of the Arithmetic subscale of the Wechsler Adult Intelligence Scale- Revised (WAIS-R: Wechsler, 1981) and four from the ArithmeticalOperations task of Demetriou, Pachaury, Metallidou, and Kazi (1996).The items were first coded dichotomously as correct or incorrect andthe maximum number of correct items was nine. A mean for thepercentage of correctly solved items was calculated separately for thetwo task types, and they were then averaged once more to produce ascore for mathematical thinking.Reasoning skills were measured by the water-level task of Piagetand Inhelder (1956). The students were asked to draw to a picture ofeight empty bottles the lines indicating the water level and mark thearea filled with water when the bottles were half full. The bottles werecoded dichotomously as correct or incorrect and a mean for thepercentage of correctly solved items was calculated.