If you’ve read our feature on the research behind video-based learning, here’s a simple rule of thumb: Keep videos short if you want to keep students watching.⁣

In 2014, researchers analyzed online lectures at MIT, Harvard, and Berkeley—constituting a whopping total of 6.9 million viewing sessions—to determine which factors improve student engagement. With a veritable mountain of evidence behind them, they concluded that the “median [student] engagement time is at most 6 minutes,” and that video length was “by far the most significant indicator of engagement,” easily beating out characteristics like production quality and instructor presence.⁣

For the longest videos—think 30-minute online lectures—students only stuck around for about 20% of the instruction time.⁣

The researchers suggest several tips for optimizing your videos:⁣

• Chunk lessons into smaller parts, using the 6-minute benchmark as a maximum duration.⁣

• Show your face, even if only for a moment. It provides a more “intimate and personal feel,” and helps to break up what can quickly become a monotonous presentation.⁣

• Opt for Khan-style tutorials—a more informal, step-by-step sketch of a problem and possible solutions—over prepared slides.⁣

• Speak normally and with enthusiasm. “There is no need to artificially slow down,” the researchers say. Students may pick up on your energy and pay more attention.