Mixtape-Exercises-Violations of PT
2022-06-23
Introduction
This exercise will walk you through using the HonestDiD R or Stata package to conduct sensitivity analysis for possible violations of parallel trends, using the methods proposed in Rambachan and Roth (2022). Here are links to the Stata package and R package.
0. Install packages if needed
We will use several Stata packages in our analysis, which you can install as follows if needed.
* reghdfe
ssc install reghdfe
* honestdid
net install honestdid, from("https://raw.githubusercontent.com/mcaceresb/stata-honestdid/main") replace
honestdid _plugin_check
* csdid
net install csdid, from ("https://raw.githubusercontent.com/friosavila/csdid_drdid/main/code/") replace1. Run the baseline DiD
For simplicity, we will first focus on assessing sensitivity to violations of parallel trends in a non-staggered DiD. Load the same dataset on Medicaid as in the previous exercise. Restrict the sample to the years 2015 and earlier, drop the small number of units who are first treated in 2015. We are now left with a panel dataset where some units are first treated in 2014 and the remaining units are not treated during the sample period.
use "https://raw.githubusercontent.com/Mixtape-Sessions/Advanced-DID/main/Exercises/Data/ehec_data.dta", clear
* Keep years before 2016. Drop the 2016 cohort
keep if (year < 2016) & (missing(yexp2) | (yexp2 != 2015))
* Create a treatment dummy
gen D = (yexp2 == 2014)
gen Dyear = cond(D, year, 2013)
(208 observations deleted)
Start by running the simple TWFE regression \(Y_{it} = \alpha_i + \lambda_t + \sum_{s \neq 2013}
1[s=t] \times D_i \times \beta_s + u_{it} ,\) where \(D_i =1\) if a unit is first treated in 2014
and 0 otherwise. Note that since we do not have staggered treatment, the
coefficients \(\hat{\beta}_s\) are
equivalent to DiD estimates between the treated and non-treated units
between period \(s\) and 2013. I
recommend using the feols command from the
fixest package in R and reghdfe command in
Stata; although feel free to use your favorite regression command. Don’t
forget to cluster your SEs at the state level.
* Run the TWFE spec
reghdfe dins b2013.Dyear, absorb(stfips year) cluster(stfips) noconstant
coefplot, vertical yline(0) ciopts(recast(rcap)) xlabel(,angle(45)) ytitle("Estimate and 95% Conf. Int.") title("Effect on dins")(MWFE estimator converged in 2 iterations)
HDFE Linear regression Number of obs = 344
Absorbing 2 HDFE groups F( 7, 42) = 10.15
Statistics robust to heteroskedasticity Prob > F = 0.0000
R-squared = 0.9464
Adj R-squared = 0.9357
Within R-sq. = 0.3726
Number of clusters (stfips) = 43 Root MSE = 0.0194
(Std. Err. adjusted for 43 clusters in stfips)
------------------------------------------------------------------------------
| Robust
dins | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
Dyear |
2008 | -.0052854 .0086536 -0.61 0.545 -.022749 .0121783
2009 | -.0112973 .0085244 -1.33 0.192 -.0285002 .0059056
2010 | -.002676 .0071078 -0.38 0.708 -.0170201 .0116681
2011 | -.0014193 .0063271 -0.22 0.824 -.0141879 .0113493
2012 | .0003397 .007391 0.05 0.964 -.0145759 .0152553
2014 | .0464469 .0091519 5.08 0.000 .0279776 .0649161
2015 | .0692062 .01035 6.69 0.000 .0483189 .0900934
------------------------------------------------------------------------------
Absorbed degrees of freedom:
-----------------------------------------------------+
Absorbed FE | Categories - Redundant = Num. Coefs |
-------------+---------------------------------------|
stfips | 43 43 0 *|
year | 8 0 8 |
-----------------------------------------------------+
* = FE nested within cluster; treated as redundant for DoF computation
3. Sensitivity analysis using relative magnitudes restrictions
We are now ready to apply the HonestDiD package to do sensitivity analysis. Suppose we’re interested in assessing the sensitivity of the estimate for 2014 (the first year of treatment). We will use the “relative magnitudes” restriction that allows the violation of parallel trends between 2013 and 2014 to be no more than \(\bar{M}\) times larger than the worst pre-treatment violation of parallel trends.
To create a sensitivity analysis, use the honestdid
function. You will need to pass the options pre and
post to specify the pre and post treatment estimates. I
suggest that you also give the optional parameter mvec a
value of 0.5(0.5)2 to specify the values of \(\bar{M}\) you wish to use. (Note: it may
take a couple of minutes to calculate the sensitivity results.)
honestdid, pre(1/5) post(7/8) mvec(0.5(0.5)2)| M | lb | ub |
| ------- | ------ | ------ |
| . | 0.029 | 0.064 | (Original)
| 0.5000 | 0.024 | 0.067 |
| 1.0000 | 0.017 | 0.072 |
| 1.5000 | 0.008 | 0.080 |
| 2.0000 | -0.001 | 0.088 |
(method = C-LF, Delta = DeltaRM, alpha = 0.050)Look at the results of the sensitivity analysis you created. For each value of \(\bar{M}\), it gives a robust confidence interval that allows for violations of parallel trends between 2013 and 2014 to be no more than \(\bar{M}\) times the max pre-treatment violation of parallel trends. What is the “breakdown” value of \(\bar{M}\) at which we can no longer reject a null effect? Interpret this parameter.
4. Create a sensitivity analysis plot
We can also visualize the sensitivity analysis using the
honestdid command by adding the coefplot
option. You can use the cached option to use the results
from the previous honestdid call (for speed’s sake).
honestdid, pre(1/5) post(7/8) mvec(0.5(0.5)2) coefplot xtitle("M") ytitle("95% Robust CI")| M | lb | ub |
| ------- | ------ | ------ |
| . | 0.029 | 0.064 | (Original)
| 0.5000 | 0.024 | 0.067 |
| 1.0000 | 0.017 | 0.072 |
| 1.5000 | 0.008 | 0.080 |
| 2.0000 | -0.001 | 0.088 |
(method = C-LF, Delta = DeltaRM, alpha = 0.050)
5. Sensitivity Analysis Using Smoothness Bounds
We can also do a sensitivity analysis based on different restrictions on what violations of parallel trends might look like. The starting point for this analysis is that often if we’re worried about violations of parallel trends, we let treated units be on a different time-trend relative to untreated units. Rambachan and Roth consider a sensitivity analysis based on this idea – how much would the difference in trends need to differ from linearity to violate a particular result? Specifically, they introduce a parameter \(M\) that says that the change in the slope of the trend can be no more than \(M\) between consecutive periods.
To create a sensitivity analysis using smoothness bounds, add the
delta(sd) option to your honestdid function
call. (Note: as before it may take a couple of minutes for the
sensitivity code to run.) What is the breakdown value of \(M\) – that is, how non-linear would the
difference in trends have to be for us not to reject a significant
effect?
honestdid, pre(1/5) post(6/7) mvec(0(0.01)0.05) delta(sd) omit coefplot xtitle("M") ytitle("95% Robust CI")| M | lb | ub |
| ------- | ------ | ------ |
| . | 0.029 | 0.064 | (Original)
| 0.0000 | 0.026 | 0.061 |
| 0.0100 | 0.013 | 0.079 |
| 0.0200 | 0.003 | 0.091 |
| 0.0300 | -0.007 | 0.101 |
| 0.0400 | -0.017 | 0.111 |
| 0.0500 | -0.027 | 0.121 |
(method = FLCI, Delta = DeltaSD, alpha = 0.050)
6. Bonus: Sensitivity Analysis for Average Effects
Re-run the sensitivity analyses above using the option
l_vec to do sensitivity on the average effect
between 2014 and 2015 rather than the effect for 2014. To do so, run the
following matrix l_vec = 0.5 \ 0.5 and then add
l_vec(l_vec) to the honestdid call
(matrix l_vec = 0 \ 1 would give inference on the 2015
effect). How do the breakdown values of \(\bar{M}\) and \(M\) compare to those for the effect in
2014? [Hint: breakdown values for longer-run effects often tend to be
smaller, since this leaves more time for the groups’ trends to diverge
from each other.]
matrix l_vec = 0.5 \ 0.5
local plotopts xtitle(Mbar) ytitle(95% Robust CI)
honestdid, l_vec(l_vec) pre(1/5) post(6/7) mvec(0(0.5)2) omit coefplot xtitle("M") ytitle("95% Robust CI")Warning: M = 0 with Delta^RM imposes exact parallel trends in the
post-treatment period, even if pre-treatment parallel trends is violated
| M | lb | ub |
| ------- | ------ | ------ |
| . | 0.040 | 0.075 | (Original)
| 0.0000 | 0.041 | 0.075 |
| 0.5000 | 0.033 | 0.080 |
| 1.0000 | 0.020 | 0.090 |
| 1.5000 | 0.006 | 0.103 |
| 2.0000 | -0.008 | 0.117 |
(method = C-LF, Delta = DeltaRM, alpha = 0.050)