Difference of a variable with time and id

[Zain Mian]

Hi
My data is of the following form:

* Example generated by -dataex-. To install: ssc install dataex
clear
input float(month id return)
555  1 .00057206413
555  2   -.07322104
555  3   -.06859436
555  4   -.07735524
555  5   -.03938038
555  6   .007824589
555  7   -.04908807
555  8   .003092327
555  9   -.10695495
555 10   -.04521626
556  1   -.04298264
556  2   -.05878122
556  3  -.033808254
556  4  -.011589252
556  5  -.017557062
556  6  -.004073501
556  7   .001721355
556  8  -.010957217
556  9   .015607742
556 10  -.013908166
557  1  -.010737965
557  2  -.018535594
557  3    .03754317
557  4   -.07786346
557  5  -.074161746
557  6   -.05729289
557  7   -.12252308
557  8    -.0617717
557  9  -.020674044
557 10   -.03216382
558  1    .03763035
558  2   .009898664
558  3   .024982596
558  4   .027306067
558  5    .02957879
558  6    .02146479
558  7   .032690376
558  8    .02783219
558  9    .02452107
558 10    .03302523
559  1    .02270367
559  2  .0017711997
559  3   .014527231
559  4   .017970225
559  5   .007367407
559  6   .031037053
559  7   .028186474
559  8   .023721296
559  9   .019167695
559 10   .018264148
560  1    .04948542
560  2    .03846701
560  3    .02795666
560  4    .02451953
560  5   .037013665
560  6   .034662504
560  7    .04399181
560  8    .04507814
560  9    .04191253
560 10     .0580038
561  1   .019825436
561  2   .032455444
561  3    .03048469
561  4    .01998458
561  5   .011587504
561  6    .02427252
561  7    .03101354
561  8   .015491064
561  9    .01145711
561 10   .017419985
562  1   .003733994
562  2  -.002126549
562  3 -.0025371325
562  4   .015725277
562  5  -.005115598
562  6   .006009568
562  7   .004166465
562  8   .011407542
562  9   .004518013
562 10   .002893288
563  1    .01868514
563  2   .014283576
563  3    .01902623
563  4   .012530405
563  5   .021287354
563  6   .008199372
563  7    .01771125
563  8   .019615415
563  9    .01664819
563 10  -.006281366
564  1  -.004248609
564  2  .0010157756
564  3  -.007604746
564  4    .01184316
564  5 .00020535664
564  6  -.003318424
564  7  -.007787342
564  8   .005721211
564  9  -.004952917
564 10   .006883996
end
format %tm month

For every month (I guess the months have been converted to Stata numbers, when I pasted them) I want to take the difference in returns between stocks with id 10 and id 1. Would appreciate if someone could help me out in taking the difference. Thank You.

[Clyde Schechter]

Assuming that in your real data the id's also really run from 1 to 10 with no gaps:

by month (id), sort: gen wanted = return[10] - return[1]

(I guess the months have been converted to Stata numbers, when I pasted them)

No, nothing has been converted to numbers. That variable has always been those numbers in your data. It is the %tm format applied to them that causes them to look like monthly dates to your eye when you -list- or -browse- them. But within Stata they are, and have always been, those numbers. This is exactly how Stata represents date and time variables. Read -help datetime- and -help datetime display formats-.

[Zain Mian]

Thanks Clyde for the help. I probably didn't realized that it was this simple. Thank You again.