Difference in differences (DID) before and after an event

[Mada Gram]

Hi everyone,
I'm new to stata. I was trying to do a diff-in-diff test for a list of firms before 2005 and after 2005 as there is an even accused which I think changed the firms' performance. the code I found is (diff depvar, t(treatmentgroup) p(postevent)
the problem I don't have treatment and control groups
Q1: are all firms before the event are considered control and after the event are treated? if I used it like this the did results are empty. or each firm is compared to itself before and after 2005?
Q2: is there another code to test for DiD?

DIFFERENCE-IN-DIFFERENCES ESTIMATION RESULTS
Number of observations in the DIFF-IN-DIFF: 471279
Before After
Control: 166330 0 166330
Treated: 0 304949 304949
166330 304949

Outcome var. depvar S. Err. t P>t

Before
Control 0.550
Treated 0.558
Diff (T-C) 0.008 0.001 9.28 0.000***
After
Control 0.558
Treated 0.566
Diff (T-C) 0.008 0.001 9.28 0.000***

Diff-in-Diff 0.000 . . .

R-square: 0.36
* Means and Standard Errors are estimated by linear regression
**Inference: *** p<0.01; ** p<0.05; * p<0.1

Thanks for your help

[Clyde Schechter]

If there is no control group, it is not possible to do a difference-in-differences analysis. Either you must find a suitable control group, or you must use a weaker design that is not as good for identifying causal effects, such as a simple pre-post comparison.

Specifically, your designation of all firms as controls before the event and treatment after does not overcome this problem and the results you show are not valid.