There is a tremendous amount of research on the effectiveness of retargeting. This is dubbed "incrementality" in the field. Issues are: 1. Incrementality is

There is a tremendous amount of research on the effectiveness of retargeting. This is dubbed "incrementality" in the field. Issues are: 1. Incrementality is extremely hard to measure due to noise and biases, in particular when trying to account for the multiple agents in the game. Attributed sales are easy to measure. For global, incremental sales, it is much harder. 2. Because of 1, this research needs large amounts of data, which only large ad tech companies have access to. 3. Because the topic is so hard to pin down and so sensitive business-wise, no private ad-tech company would publish ground-breaking material on the topic.