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ddtiming is a Stata command that implements a decomposition of a difference-in-differences (DD) estimator with variation in treatment timing, based on Goodman-Bacon (2021). The two-way fixed effects DD model is a weighted average of all possible two-group/two period DD estimators. The command generates a scatterplot of 2x2 DD estimates and their associated weights.
An R package called bacondecomp is available to perform the decomposition.
ddtiming is not under active development. Stata users should instead use bacondecomp, a more recent implementation of the Bacon decomposition in Stata.
To install the
ddtiming command in Stata, either type into the command window
net describe ddtiming, from(https://tgoldring.com/code)
net install ddtiming
net install ddtiming, from(https://tgoldring.com/code)
Example – No-Fault Divorce Laws
ddtiming can replicate the example in Goodman-Bacon (2021). Download and load a dataset with the timing of no-fault divorce laws and female suicide rates (Stevenson & Wolfers, 2006):
net get ddtiming
For comparison, estimate a two-way fixed effects DD model of female suicide on no-fault divorce reforms:
areg asmrs treat i.year, a(state) vce(robust)
Apply the DD decomposition theorem in Goodman-Bacon (2018) to the two-way fixed effects DD model:
ddtiming asmrs treat, i(state) t(year)
Stata will produce DD estimates, the associated weights, and a scatterplot of the estimates. The scatterplot replicates Fig. 6 in Goodman-Bacon (2021). Additionally, we can add options to the command to modify the look of the scatterplot:
ddtiming asmrs treat, i(state) t(year) ddline(lwidth(thick)) ///
ylabel(-30(10)30) legend(order(3 4 1 2)) savegraph(nfd.jpg) ///
This command demonstrates the use of
ddtiming's options (
savedata) and twoway options (
legend). For descriptions of all options and additional help, type
Goldring, T. (2019). ddtiming: Stata module to perform a Goodman-Bacon decomposition of difference-in-differences estimation. tgoldring.github.io/projects/ddtiming
Goodman-Bacon, A. (2021). Difference-in-differences with variation in treatment timing. Journal of Econometrics, 225(2), 254–277.
Stevenson, B., & Wolfers, J. (2006). Bargaining in the Shadow of the Law: Divorce Laws and Family Distress. The Quarterly Journal of Economics, 121(1): 267–288.
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