Previous research on curriculum-based measurement of oral reading fluency (CBM ORF) found high levels of variability around the estimates of students' fluency; however, little research has studied the issue of variability specifically with well-designed passage sets and a sample of students who scored below benchmark for the purpose of progress monitoring. We examined the variability in oral reading fluency score slopes due to passage and student characteristics using "DIBELS Next" progress monitoring passages over 4 weeks using a hierarchical linear growth model. Participants included second-, third-, and fourth-grade students identified as at risk for reading difficulties. The results showed an average growth rate of approximately 1 correctly read word per minute per week, with considerably lower variability than shown in previous research with less controlled passage sets and/or higher performing student samples. Implications for practice are discussed, including procedural recommendations for administrators and teachers.