CSDID estimation sample smaller than full panel by exactly N individuals — expected behavior?

[YITING HUANG]

Dear Statalist,

I am using csdid2 (Callaway & Sant'Anna 2021) with a balanced panel of N = 753,050 individuals observed over 13 years (2011–2023).
All individuals are eventually treated (first childbirth between 2017–2023); I use the not-yet-treated as the control group (notyet option). The base period is g-1 (universal base, default).

After estimation, the reported "Number of obs" is 9,036,600, which is exactly 753,050 × 12 — one fewer year per individual than the full balanced panel (9,789,650 = 753,050 × 13).

My understanding is that the base period (t = -1, i.e., g-1 for each individual) is used as the reference for first-differencing and therefore does not count as an independent estimation period, reducing the reported N by one observation per individual.

Is this the correct explanation for the discrepancy? Is there any documentation or reference that explicitly describes this behavior?

I note that a similar pattern appears in published work using csdid with the same base period convention, where t = -1 is omitted from the event-study table entirely, consistent with this interpretation.

Thank you in advance.

[Jeff Wooldridge]

Yiting: The number reported is due to having no never treated individuals. The way both csdid and jwdid work is that no effects are estimated in the final time period and so that time period is dropped. I actually prefer to estimate effects comparing the earlier treated cohorts to the final treated cohort, but none of the special commands do that. It doesn't change the estimates in the earlier periods. So, you can use all 13 years, but in the final year the estimated effects would compare cohorts 2017 through 2022 with 2023 in the year 2023. Of course you can't estimate an effect for the final cohort. I can send a do file that shows how to do this "by hand" using reg or xtreg.