Recently, Nissen et al. argued in this journal for the use of Cohen's d, in place of the more commonly used normalized gain, in the analysis of preinstruction and postinstruction scores on concept inventories used to measure the effectiveness of instruction. Their reason for advocating such a change is that they say normalized gains are "prescore biased." We provide five examples, including one cited by Nissen, that show no prescore bias when data are carefully analyzed, demonstrating that the problem with their analysis is omitted variable bias. We show that Cohen's d is less informative than normalized gain when used as a single parameter measure of teaching effectiveness, even though, as Nissen points out, d is more widely used in other fields. We believe that physics education researchers should continue to use normalized gain to assess educational effectiveness of pedagogy. However, because different student populations can have significantly different responses to the same pedagogy, in any interpretation of normalized gain, it is important to consider a measure of the abilities of the students. In analyzing normalized gains for the Force Concept Inventory (FCI), average scores on either Lawson's Test of Scientific Reasoning Ability or the SAT should be considered, because these scores are strongly correlated with normalized gain, indicating student abilities may have a greater impact on the gains achieved in a class than the specific pedagogy used.