Instruction plays a critical role in student success. However, most studies of teachers' effects on student learning focus on face-to-face settings. Some aspects of online courses, such as the choice of synchronous or asynchronous instruction and the course structure, might reduce teachers' influence on their students. For example, online courses that are highly prescriptive, with teachers using the same materials, assignments, and assessments, might lessen online teachers' influence on student outcomes. By reducing the variation in teachers' instructional practices, highly structured online courses might also reduce differences in student performance across teachers compared with face-to-face instruction. This study of student outcomes for segment 1 (similar to a semester) of an online high school biology course offered by Florida Virtual School examined the amount of variation in segment completion rates, students' end-of-segment exam scores, and time to completion that is attributable to teachers. Determining teachers' influence on students' course outcomes requires separating the influence of differences in teachers' instruction from the influence of differences in student characteristics. The analyses in this study looked at the total variation across students' course outcomes and attributed it to differences between teachers or between students. [For the full report, see ED609863; for the appendixes, see ED609865.]