Dear all,

I have encountered a problem when using propensity score matching to match observations in the concurrent year with observations in the previous year.

I have tried the following code:

psmatch2 X_main control_1 control_2 if year==1981, out(Y) common

(X_main is the key explanatory variables, Y is the outcome variable, the rest are control variables)

I thought the year specification would match all observations in the data-set with observations in the year 1981, but it turns out that observations in other years are also included in the _id column.

I have also tried this code:

psmatch2 X_main control_1 control_2 in 1/500, out(Y) common

I thought the in 1/500 would match all observations with the 1st to 500th observations in the data-set, but it turns out that only the first 500 observations are matched with each other.

Could anyone kindly teach me how to match all observations in the data-set (whole sample) with observations in a given year (or a given range)/ subsample?

Thank you so much!