Difference-in-differences parallel trends for treatment and control groups of uneven size I am confused about how to run a formal parallel trends test for my DiD study. Here is an overview of my data.
There are 25 districts in the treatment group. Each district has data on district-level average income for five periods (P1, P2, P3, P4, P5) each year from 1900 to 1910.
There are 60 districts in the control group. As in treatment, each of the 60 districts have district-level average income for five periods  each year from 1900 to 1910.
The intervention happened in 1905. I want to estimate its effect on the income at the district-level using difference-in-differences. To this end, I want to first ensure that parallel trends assumption is fulfilled. Namely, I need to test that evolution of income in the treatment group would have followed the same trend as in control group if there was no intervention.
I have read several posts about parallel trends on this forum, but confusion does not go away. I read a paper that does a t-test (to compare the means of two groups) as a formal test for parallel trends assumption. Not sure I can simply average  the incomes of all districts in both treatment and control groups for each time period. This way I think I can also do a t-test. I do not know if this way of averaging is a good idea in my case. Any suggestions?

In case someone will later have a similar question on testing parallel trends assumption, I want to share my understanding.
PT assumption cannot be fundamentally tested because we don't observe the counterfactuals of how treated group would evolve without the treatment. So, what people do in practice is test the pre-trends: whether treatment and control groups evolved similarly before the actual treatment. This does not validate PT assumption in full generality but does provide a suggestive evidence. To that end, there are at least three tests one can do. They are discussed in details with codes in the wonderful textbook of Nick Huntington-Klein, freely available here: https://www.theeffectbook.net/ch-DifferenceinDifference.html
