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.

ddtiming is not under active development. Stata users should instead use bacondecomp, a more recent implementation of the Bacon decomposition in Stata.

An R package called bacondecomp is available to perform the decomposition.

Type the following command:

net install ddtiming, from(https://raw.githubusercontent.com/tgoldring/ddtiming/master)

Alternatively, you can download the package from GitHub and install it manually.

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, from(https://raw.githubusercontent.com/tgoldring/ddtiming/master)

use nofault_divorce.dta

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 (2021) 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) ///
savedata(nfd) replace

This command demonstrates the use of ddtiming's options (ddline, savegraph, savedata, and twoway options ylabel and legend). For descriptions of all options and additional help, type

help ddtiming

Please cite ddtiming as:

Goldring, T. (2019). ddtiming: Stata module to perform a Goodman-Bacon decomposition of difference-in-differences estimation. https://github.com/tgoldring/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.