Within the past decade, there has been a growing number of studies examining undermatch--when students apply to or enroll in institutions less selective than their academic qualifications permit. To estimate undermatch, researchers must define institutions' selectivity levels and determine which students are eligible to gain admission to these selectivity levels. Researchers examining undermatch have used different approaches to defining institutional selectivity and student qualifications. This, in turn, has produced a wide range of undermatch rates, and at times, conflicting or inconclusive findings for underrepresented students. As the body of literature on undermatch expands, the tradeoffs and limitations in estimation approaches must be better understood. Using a nationally representative sample of students (ELS:2002), this study empirically tested these differences in undermatch estimations using two different definitions of institutional selectivity and three distinct approaches of calculating student qualifications on the (1) distribution of students across qualification levels; (2) undermatch rates; and (3) likelihood of undermatch. Findings show that depending on the approach taken, the distribution of student qualifications, undermatch rates, and odds ratios in subsequent analyses can vary greatly. For underrepresented students, the difference in estimation methods can change their representation in various qualification levels, the gaps in undermatch rates, and the significance of results in their likelihood of undermatching. Implications for future undermatch research are discussed.