Measuring the effect of Napster on recorded music sales: difference-in-differences estimates under compositional changes

This paper measures the effect of Napster on record sales. I treat the introduction of Napster as a technological event that only Internet users experienced, and use a difference-in-differences (DD) approach. Because of potential compositional changes in Internet users, I examine identifying assumptions for the DD estimator under compositional changes and develop a test for identifying restrictions. To address potential bias due to compositional changes, I extend DD matching estimators to the case of two-variate propensity scores. I find evidence suggesting that file sharing is likely to explain 20% of total sales decline, which is driven by households with children aged 6—17.