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Online platforms are prone to abuse and manipulations from strategic parties. For example, social media and review websites suffer from sentiment manipulations, manifested in the form of opinion spam and fake reviews. The consequence of such manipulations is the deterioration of information quality as well as loss in consumer welfare. We study one of movie studios' operation activities, sentiment manipulation, in the context of movie tweets. Using the movie release and movie studios' earning announcement dates as sources of exogenous shocks, we find that both the average Twitter sentiment and the proportion of highly positive tweets exhibit a significant drop on the movie's release day or movie studios' earnings announcement day. In addition, independent productions and low budget movies tend to experience a larger drop than major studio productions and high budget movies. To examine the effect of …