We analyze an experiment in which middle-school math teachers were randomly given access to "off-the-shelf" lessons designed to develop students' deep understanding. These lessons were provided online, but are designed to be taught by teachers in a traditional classroom setting. Teaching involves multiple complementary tasks, but we model two: imparting knowledge and developing understanding. In our model, lessons designed to develop understanding substitute for teacher effort on this task so that teachers who may only excel at imparting knowledge can be effective overall - simplifying the job of teaching. Providing teachers with online access to the lessons with supports to promote their use increased students' math achievement by about 0.08 of a standard deviation. These effects appear to be mediated by the lessons promoting deep understanding, and teachers therefore being able to provide more individualized attention. Benefits were much larger for weaker teachers, suggesting that weaker teachers compensated for skill deficiencies by substituting the lessons for their own efforts. The intervention is highly scalable and is more cost effective than most policies aimed at improving teacher quality.
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