I used attnd command to estimate nearest-neighbor matching. Then, I got the result as follows.


*ATT estimation with Nearest Neighbor Matching method
*(random draw version)
*Analytical standard errors

*---------------------------------------------------------
*n. treat. n. contr. ATT Std. Err. t
*---------------------------------------------------------

* 69 42 0.667 0.160 4.175

*---------------------------------------------------------
*Note: the numbers of treated and controls refer to actual
*nearest neighbour matches


I am curious why the number of observations differs between treatment (69) and control (42). I thought the number of observations balance as it performs matching.


I also checked the STATA seminar document and then found out the document's result also shows the different observation numbers.
https://www.stata.com/meeting/italy1...t14_grotta.pdf

Does anybody know why the observations differ?