where zc is the per capita cumulative spending in the program from April 1933 to December 1935 in county c, rc is the share of households in county c with a radio, tc denotes voter turnout in gubernatorial elections in county c, xc is a set of control variables, μs are state fixed-effects, and εc is an error term uncorrelated with the regressors. According to the model, c1 > 0 and 0 < c2 < 1.
state fixed-effects