I have been working on trying to find whether there has been a positive impact on the voting share of Democratic candidates for governor following the expansion of Medicaid in 2014 using the diff-in-diff technique. My control group is made up of two states that did not adopt the expansion, Florida and Wisconsin, while the treatment group consists of Colorado and Minnesota. The elections I consider were held in 2006 and 2010 (pre-treatment) and in 2014 and 2018 (post-treatment: the expansion took effect in January of 2014). My data is at the county level.
My question is whether I should be considering the results obtained using the command regress or teffects.
For clarity: the "expansion" variable stands for whether a state implemented the expansion or not, while "post" stands for whether the year is after 2014 or not. So expansion#post (in the first regression) and postexp (in the second) are my diff-in-diff terms.
Code:
reg voteshare_dem expansion#post insured_perc unemployment_rate Poverty_rate Median_income Black_perc Hisp_perc Na > tive_perc Asian_perc Pop_dens nationalgains_dem previous_pres_dem [aweight=pop_total], cl(state) (sum of wgt is 117,873,853) Linear regression Number of obs = 1,160 F(2, 3) = . Prob > F = . R-squared = 0.8780 Root MSE = 4.1274 (Std. Err. adjusted for 4 clusters in state) ----------------------------------------------------------------------------------- | Robust voteshare_dem | Coef. Std. Err. t P>|t| [95% Conf. Interval] ------------------+---------------------------------------------------------------- expansion#post | 0 1 | 3.077134 1.164534 2.64 0.077 -.6289329 6.7832 1 0 | 2.362998 4.430529 0.53 0.631 -11.73692 16.46292 1 1 | 8.679507 2.622158 3.31 0.045 .3346303 17.02438 | insured_perc | -.2027824 .1108363 -1.83 0.165 -.5555131 .1499482 unemployment_rate | .748344 .520117 1.44 0.246 -.9069005 2.403588 Poverty_rate | .2958827 .1199389 2.47 0.090 -.0858166 .6775819 Median_income | .0000794 9.91e-06 8.01 0.004 .0000478 .0001109 Black_perc | .0356887 .0800048 0.45 0.686 -.2189222 .2902997 Hisp_perc | -.0426906 .0055727 -7.66 0.005 -.0604254 -.0249559 Native_perc | -.0281068 .0396139 -0.71 0.529 -.1541759 .0979623 Asian_perc | -.0856813 .1318648 -0.65 0.562 -.5053339 .3339712 Pop_dens | -.00026 .0003716 -0.70 0.535 -.0014427 .0009228 nationalgains_dem | 8.594712 2.096184 4.10 0.026 1.92372 15.2657 previous_pres_dem | .8987743 .0491679 18.28 0.000 .7423002 1.055248 _cons | 2.827052 7.991739 0.35 0.747 -22.60623 28.26033 -----------------------------------------------------------------------------------
Code:
teffects ra (voteshare_dem insured_perc unemployment_rate Poverty_rate Median_income Black_perc Hisp_perc Native_p > erc Asian_perc Pop_dens nationalgains_dem previous_pres_dem) (postexp), vce(cluster state) Iteration 0: EE criterion = 2.316e-25 Iteration 1: EE criterion = 2.817e-29 Treatment-effects estimation Number of obs = 1,160 Estimator : regression adjustment Outcome model : linear Treatment model: none (Std. Err. adjusted for 4 clusters in state) ----------------------------------------------------------------------------------- | Robust voteshare_dem | Coef. Std. Err. z P>|z| [95% Conf. Interval] ------------------+---------------------------------------------------------------- ATE | postexp | (1 vs 0) | 3.582105 1.131778 3.17 0.002 1.363861 5.80035 ------------------+---------------------------------------------------------------- POmean | postexp | 0 | 42.52256 1.627895 26.12 0.000 39.33195 45.71318 -----------------------------------------------------------------------------------
Thank you very much in advance
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