This chapter provides an overview of student growth percentile (SGP) as a tool in formative assessment, framed within the context of English language learners achieving English language proficiency. Since SGPs compare students' performances to their peers, that is, those with an identical history of achievement, individualized SGP scores on a test could be different even for students with identical current scores. Because no vertical scale or equating is required, SGPs need less turnaround time and are less expensive to undertake. This chapter provides an example of formative assessment using quantile regression method and statistical analytical system programming to help in determining the propensity of students in achieving an SGP score that would be needed to attain a predetermined passing score in English language proficiency (ELP) and its four modalities (i.e., reading, writing, listening, and speaking).