The notion that face-to-face time with a professor is the best mode of learning is deeply ingrained in American higher education.  Despite the potential large cost savings of reducing class time and shifting some learning online, there is little experimental evidence on the causal impact of time in the classroom on academic performance.  In this paper, we test whether students in a hybrid format of introductory microeconomics, which met once per week, performed as well as students in a traditional lecture format of the same class, which met twice per week.   We randomized 725 students at a large, urban public university into the two formats, and unlike past studies, had a very high participation rate of 96 percent. Two experienced professors taught one section of each format, and students in both formats had access to the same online materials.  We find that students in the traditional format scored 2.3 percentage points more on a 100-point scale on the combined midterm and final.  There were no differences between formats in non-cognitive effort (attendance, time spent with online materials) nor in withdrawal from the class. Our results suggest that the large effects of attending lectures found in the previous literature are likely due to selection bias, and that hybrid classes may offer a cost effective alternative to traditional lectures while having a small impact on student performance.