Smoking and Lost Workdays [Problem 15.6 from Wooldridge] Consider taking a large random sample of workers at a given point in time. Let sicki =1 if person i called in sick during the last 90 days, and zero otherwise. Let zi be a vector of individual and employer characteristics. Let cigsi be the number of cigarettes individual i smokes per day (on average).

Elucidate the underlying experiment of interest when we want to examine the effects of cigarette smoking on workdays lost.