# ddtiming

`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 (2018). 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.

To install the command, either type

```
net describe ddtiming, from(https://tgoldring.com/code/)
net install ddtiming
```

or alternatively type
```
net install ddtiming, from(https://tgoldring.com/code/)
```

Let's see `ddtiming`

in action using the same example as in Goodman-Bacon (2018). Download and load a dataset with the timing of no-fault divorce laws and female suicide rates (Stevenson & Wolfers, 2018) by typing

```
net get ddtiming
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) 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)
```

You should see the following output and scatterplot:
The scatterplot replicates Figure 6 in Goodman-Bacon (2018). 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`

, `legend`

). For descriptions of all options and additional help, type
```
help ddtiming
```

# References

Goodman-Bacon, Andrew. 2018. "Difference-in-differences with variation in treatment timing". Working paper.

Stevenson, Betsey and Justin Wolfers. 2006. "Bargaining in the Shadow of the Law: Divorce Laws and Family Distress". *The Quarterly Journal of Economics* 121(1): 267-288.