The purpose of the current project was to compare two different effect size measures used in assessment of differential item functioning (DIF). Specifically, in the context of logistic regression analysis, which is a common methodology for assessing DIF, we compared the DIF decisions based on the use of ∆R² effect size measure and decisions based on use of log odds ratios (∆_LR). This problem was addressed within a study concerned with DIF of a delinquency scale commonly used in antisocial/delinquent behavior research. We examined the data collected from 3290 students in the city of Toronto (Canadian portion of international study concerned with behaviour and misbehaviour of students in grades 7 to 9; the International Self-report Delinquency Study, Enzmann et al., 2010). We evaluated DIF of the utilized delinquency scale in relation to four grouping variables relevant for delinquent behaviour: gender, age, socio-economic status, and neighbourhood context (i.e., crime in neighbourhood). According to the results, conclusions about DIF were related to the choice of effect size measure; that is, different conclusions resulted from the use of the different effect size measures. Our results, obtained by utilizing real data, were in line with the recent simulation studies that pointed towards low power of the ∆R² effect size measure in detecting DIF (Hidalgo & Lopez-Pina, 2004; Hidalgo et al., 2014). The results emphasize a need for further examination of the logistic regression effect sizes and their optimal cutoffs in regard to DIF. As DIF is of importance in development of psychological tests and measures as well as in interpretations/conclusions based on tests and measures, we discuss the results in the context of methodological choices in detecting DIF and practical consequences of such choices.