screenshot in case twit goes away 
Hal Varian explains causal inference in economics and marketing
Krueger study on restaurant that kickstarged causal inference and natural experiments in economics Minimum Wages and Employment: A Case Study of the Fast Food Industry in New Jersey and Pennsylvania
BTW I had to edit all the causal in the post since I had written casual, maybe denotes my idea on the issue 🙂
1500-Pound Goat 