I am doing a quasi experimentation and am interested in getting ATT. i have a data with 260k entries where Ti = 0 and 5k entries where Ti = 1. I am calculating ATT with iptw technique where I achieve a great balance and the treatment effect -on treated as -ve 450 euros but not significant.
Weight calculation: (If treatment = 1, weight = 1 else propensity score / (1-propensity score)
Then, to compare against other methodology, I use nearest neighbour matching with ratio = 1, the balance is again achieved. I get treatment effect (which is ATT by default in matching) as +very 750 and significant.
Shouldn't both method generate similar result ? Which method should I go for in this case and why?
When you match, is there any treated individuals without a match?
In expectation, IPTW and matching should both give the same answer. One possible explanation is that some treated individuals don't have a close match, so they are dropped. When this happens, the population for which the causal effect is defined changes. This could results in different answer between the methods