" Looking at the sales statistics is not going to help you find an answer any more than looking at the number of people recovering from bloodletting will help you find out if the treatment is effective. The reason is simple: you can’t observe the counterfactual. You don’t know whether the change in sales was caused by something else; something, perhaps, you hadn’t even considered. "

― Matthew Syed , Black Box Thinking: Why Some People Never Learn from Their Mistakes - But Some Do