The article examines how mergers in businesses that compete using both prices and promotions are predicted inaccurately when only looking at prices. For example, a merger in the super-fancy ice cream industry resulted in the U.S. Federal Trade Commission challenging the Nestlé and Dreyer's merger. By studying how promotional tactics affect competition, the research found that ignoring promotional competition when predicting merger outcomes leads to significant errors. In this case, the predicted price impacts of the Nestlé-Dreyer's merger were off by a large margin, with estimated demand being too flexible and post-merger promotional activities decreasing instead of staying the same.