Taking a test generally improves the retention of the material tested. This is a phenomenon commonly referred to as testing effect. The present research investigated whether two-tier diagnostic tests promoted student teachers' conceptual understanding of variables in conducting scientific experiments, which is a scientific process skill. In this study, 97 preservice science teachers were divided into three groups. All three groups took a physics laboratory course during the fall term in which they learned about variables in the course. Each group was engaged in a different learning activity after the physics laboratory course: taking a two-tier diagnostic test, taking a multiple choice test, or studying a lecture summary. The narratives of the participants before and after the learning activities were transcribed into a format of "flow maps" to assess their conceptual understanding of variables. The results revealed that the two-tier diagnostic test is an effective tool in promoting conceptual understanding of complex concepts about variables in conducting scientific experiments.