If you’ve ever worked in marketing you’ll hear the word “incrementality” constantly. Incrementality doesn’t care about someone who happened to see your Facebook banner ad before converting. It cares about people who converted because they saw your Facebook banner ad. It is a causal question: is marketing causing conversions? And causal questions require you to think about what would have happened had someone not seen your ad (the fancy term for this is the “counterfactual”).  What we’d really like to do is get a quantum physicist, create two parallel universes that are identical except for whether or not you showed each user an ad, and observe what happens. Unfortunately physicists have “morals”, and “strong ethical concerns” about helping you, so…