I am learning how to run a difference in difference model,
and my code is
areg y treat1996-evergm1999 i.year, absorb(id)
I plot the coefficient against year and the coefficient is the difference in y between the treat and control group


suppose my previous data look like this, very balanced
id year y treated or not treat1996 treat1997 treat1998 treat1999
1 1996 250 1 1 0 0 0
1 1997 132 1 0 1 0 0
1 1998 131 1 0 0 1 0
1 1999 175 1 0 0 0 1
2 1996 165 0 0 0 0 0
2 1997 242 0 0 0 0 0
2 1998 231 0 0 0 0 0
2 1999 213 0 0 0 0 0
3 1996 165 1 1 0 0 0
3 1997 242 1 0 1 0 0
3 1998 231 1 0 0 1 0
3 1999 213 1 0 0 0 1
4 1996 165 0 0 0 0 0
4 1997 242 0 0 0 0 0
4 1998 231 0 0 0 0 0
4 1999 213 0 0 0 0 0

but then i deleted 2 obs (marked as red)
id year y treated or not treat1996 treat1997 treat1998 treat1999
1 1996 250 1 1 0 0 0
1 1997 132 1 0 1 0 0
1 1998 131 1 0 0 1 0
1 1999 175 1 0 0 0 1
2 1996 165 0 0 0 0 0
2 1997 242 0 0 0 0 0
2 1998 231 0 0 0 0 0
2 1999 213 0 0 0 0 0
3 1996 165 1 1 0 0 0
3 1997 242 1 0 1 0 0
3 1998 231 1 0 0 1 0
3 1999 213 1 0 0 0 1
4 1996 165 0 0 0 0 0
4 1997 242 0 0 0 0 0
4 1998 231 0 0 0 0 0
4 1999 213 0 0 0 0 0


and i run the above code again, the coefficients changed (which is not surprising)
but i dont know what did Stata do in this regression
i furthur run the code and find no obs were deleted from the second regression.
tab id if e(sample)
tab id if !e(sample)


. tab id if !e(sample)
no observations

then, why the coefficient is different??