Introduction to Econometrics 3rd Edition by James Stock, Mark Watson

Exercise 6

Suppose that you have just read a careful statistical study of the effect of advertising on the demand for cigarettes. Using data from New York during the 1970s, the study concluded that advertising on buses and subways was more effective than print advertising. Use the concept of external validity to determine if these results are likely to apply to Boston in the 1970s; Los Angeles in the 1970s; New York in 2010.