In this study, we identified middle school and college students' prior ideas about electrostatic induction and interviewed them through presenting observational evidence which supported or refuted their own prior ideas. Their responses to the evidence were interpreted from perspectives based on philosophies of science, especially the Popperian and Lakatosian views of scientific hypothesis testing. In the process of confirmation, almost all of the students showed a logical error known as an 'error of affirmation of the consequent' in a syllogism. The students' falsification processes were classified into two groups: those which rejected the hard core of prior ideas, and those which modified the students' protective belt of auxiliary ideas related to the hard core while still preserving the hard core. From an analysis of the students' falsification processes, it was found that the Lakatosian rather than the Popperian view was more acceptable in understanding the students' responses to the conflicting evidence. It was observed that the quality of the understanding of auxiliary ideas should also play an important role in the changing of core concepts.