Get random sample of x% of each stratum

*(Q) How can I select x% of cases in each stratum. I do not know the number of cases
	within each startum nor the total number of cases.

*(A) Emailed to the person who asked the question in Dec 2001.
* 	by rlevesque@videotron.ca.


* Construct a dummy data file for illustration purposes.
NEW FILE.
INPUT PROGRAM.
LOOP H=1 TO 10.
* assume the minimum number of cases in a stratum (hn) is 15.
COMPUTE hn=15 + TRUNC(UNIFORM(25)).
LEAVE h hn.
LOOP id=1 TO hn.
+	COMPUTE val=UNIFORM(100).
+	END CASE.
END LOOP.
END LOOP.
END FILE.
END INPUT PROGRAM.
LIST.

FREQ VAR=hn.

SET MPRINT=no.

* Start the job.
* (You would replace the above code by your own data using a GET FILE command)

* Define a macro which will do the job.
*////////////////////////.
DEFINE !sample (hvar=!TOKENS(1) /frac=!TOKENS(1))
/* Calculate the number of cases per stratum*/.
/* (I assume the variable hn does not exist)*/.
COMPUTE nobreak=1.
RANK  VARIABLES=nobreak BY !hvar  /N INTO hn2.

* Calculate the population size (N).
RANK  VARIABLES=!hvar BY nobreak  /N INTO n.

* Calculate required sample size (ssize) in each strata (rounded to nearest integer).
COMPUTE ssize=RND(!frac*hn2).

COMPUTE draw=UNIFORM(1).
RANK
  VARIABLES=draw BY !hvar  /RANK INTO rdraw.
EXECUTE.
* Select the sample.
SELECT IF (rdraw <= ssize).
!ENDDEFINE.
*////////////////////////.

* Use the macro.
* Here strata variable is called h and 20% of cases are required.
SET MPRINT=yes.
!sample hvar=h frac=.2.

* Next line is to facilitate checking that results are ok.
SORT CASES BY h rdraw.