Although an important goal of introductory physics labs is to train students in scientific reasoning and critical thinking, currently there are no standard tests in physics designed to assess such skills. We are in the process of developing and validating the Physics Lab Inventory of Critical thinking (PLIC), an assessment to probe students' critical thinking abilities in physics lab courses. The instrument asks students to critique a set of experimental methods and data and use them to evaluate a particular physical model (the period of a mass on a spring). Currently, we are validating the closed-response survey through interviews with students and present the results of 12 such interviews here. We describe a trend that has emerged from these interviews, with students' reasoning falling into three main patterns of behavior: selecting all options, cuing on keywords, and critically analyzing. We have found ways to shift students to the last and more desirable behavior. We discuss ways in which these findings are likely relevant to the design of other concept inventories.