This study examined whether endogeneity exists between firms that receive non-audit services from the same auditor and those that do not, and analyzed whether there are differences in audit quality, audit fees, and audit hours after controlling for endogeneity using the before-and-after comparison method. Additionally, it investigated whether there are differences from the empirical results obtained using propensity score matching and Heckman analysis, which have been primarily used in prior research for endogeneity control. The empirical analysis results of this study are as follows. First, the analysis of differences in audit fees and audit hours between firms that received non-audit services and those that did not revealed that firms receiving non-audit services had greater audit fees and audit hours after receiving non-audit services than before. This demonstrates that existing research results may have been distorted due to endogeneity. Second, when examining the relationship between non-audit services and audit fees and audit hours through the before-and-after comparison method as an endogeneity control technique, no significant differences were found. This result differs from prior research findings that firms receiving non-audit services have higher audit fees and audit hours than those that do not. In contrast, when controlling for endogeneity through propensity score matching and Heckman analysis, results similar to prior research were obtained, showing that firms receiving non-audit services had higher audit fees and audit hours than those that did not. This is significant in that it directly demonstrates that the endogeneity problem raised by Lawrence et al. (2011) can also appear in non-audit service research. The results of this study are expected to provide meaningful information to policymakers regulating non-audit services from the same auditor and to subsequent researchers.