The methodology of using two isomorphic (same form) problems focuses on detecting differences in students' performances when solving one of the problems. In this article we present a methodology that not only focuses on that, but also analyzes the effect on performance when students have previously done the associated problem, as well as the way students' answers change ("answer changing") when doing both problems, and the effect of problem order on answer changing. We administered isomorphic tests with vector problems to 330 students finishing a mechanics course. We used two sets of problems: negative scalar multiplication (no-context and velocity context) and dot product interpretation (no-context and work context). In the first set, we detected a significant effect produced by the order in which students answered the problems. In the second set, we found that a great proportion of students answered the first work context problem correctly, but incorrectly answered the second no-context problem. This article presents the methodology so it can be used by other researchers.