Two theories of assessing function knowledge were compared for intuitive physics. The choice assessment theory, derived from Piaget, presents subjects with two physical situations, each specified by the values of two physical variables; subjects choose the situation which will yield the greater value of a dependent variable. Functional measurement presents subjects with a single physical situation; subjects make a quantitative estimate of the dependent variable. Forty subjects made both choice and functional measurement responses for two situations of intuitive physics. The choice theory showed substantial frequencies of stepwise rules, implying that subjects failed to integrate the two given physical variables. Functional measurement, in contrast, showed that most subjects integrated the two variables, following exact addition or multiplication rules. It is concluded that functional measurement gives a more correct assessment of function knowledge and should be useful in science instruction.