Hegarty, M., Just, M. A. & Morrison, I. R. (1988). Mental models of mechanical systems: Individual differences in qualitative and quantitative reasoning. Cognitive Psychology, 20, 191-236.

People who understand mechanical systems can infer the principles of operation of an unfamiliar device from their knowledge of the device's components and their mechanical interactions. Individuals vary considerably in their ability to make this type of inference. This paper describes studies of performance in psychometric tests of mechanical ability. Based on subjects' retrospective protocols and response patterns, it was possible to identify rules of mechanical reasoning that accounted for the performance of subjects of different levels of mechanical ability. The rules are explicitly stated in a stimulation model which demonstrated the sufficiency of the rules by producing the kinds of responses observed in the subjects. Three abilities are proposed as the sources of individual differences in performances: (1) ability to correctly identify which attributes of a system are relevant to its mechanical function, (2) ability to use rules consistently, and (3) ability to quantitatively combine information about two or more relevant attributes.