Some students may guess at random or be inattentive in a testing situation. Several approaches have been developed to detect these types of behavior. The use of person-fit index is the most studied approach and seems very promising. In this study, we focus on three popular indices which have many features to facilitate their interpretation: lz, ZU and ZW. Nevertheless, previous studies have shown that these three indices are strongly affected by the fact that the ability of a student is estimated rather than known. Snijders (2001) proposed a corrected version of the lz index (named lz*) to take account of this problem. Magis, Béland, and Raîche (2014) have already used the Snijders correction to create two person-fit indexes: U* and W*. It is now time to extend our understanding of the corrected indexes lz*, U*, and W*, and standardized indices lz, ZU, and ZW. To do this, we conduct two studies using different values of the student’s ability: an analysis of type I errors (probability of being wrong in identifying inappropriate response patterns), and an analysis of the power of detection of theses indexes. Our results show that the corrected indices lz* and W* are most interesting because their scores are approximately normally distributed and allow to adequately detect guessing at random and inattention response patterns.