Inhelder and Piaget (1958) studied schoolchildren's understanding of a simple pendulum as a means of investigating the development of the control of variables scheme and the "ceteris paribus" principle central to scientific experimentation.The time-consuming nature of the individual interview technique used by Inhelder has led to the development of a whole range of group test techniques aimed attesting the empirical validity and increasing the practical utility of Piaget's work.The Rasch measurement techniques utilized in this study reveal that the Piagetian Reasoning Task III-Pendulum and the "methode clinique" interview reveal the same underlying ability. Of particular interest to classroom teachers is the evidence that some individuals produced rather disparate performances across the two testing situations. The implications of the commonalities and individual differences in performance for interpreting children's scientific understanding are discussed.