Proportional sampling without replace

 

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* Proportionate sampling without replacement.
*actually sampling is done with replacement but a block is retained only the first time it is selected.

*City blocks have different populations. The objective is to select a random number of blocks 
where the probability of selecting a given block is proportional to its population. 

*The 3 files 
draw file1.sav 
draw file2.sav
draw file3.sav
contain intermediary results. The sole purpose of these files is to help understand and verify what the syntax is doing.
* Raynald Levesque raynald@spsstools.net .

*************************************************************.
*Generate a file of 1000 cases for illustration purposes.
*************************************************************.
NEW file.
input program.
loop block=1 to 1000.
leave block.
compute pop=RND(uniform(10000)).
FORMATS block pop (F8.0).
end case.
end loop.
end file.
end input program.
execute.

SET MPRINT=ON.


*/////////////////////////// BEG OF MACRO//////////////////////////////.

DEFINE !SAMPLE (draw=!TOKENS(1) /keep=!TOKENS(1))

COMPUTE case# =$casenum.
CREATE t_weight=CSUM(pop).
SORT CASES BY case#(D).
CREATE c_weight= CSUM(pop).
SORT CASES BY case#.
IF $casenum=1 #tot_pop=c_weight.

VECTOR rva rvb (!draw F8.0).
!DO !cnt = 1 !TO !draw
	DO IF $casenum=1.
		COMPUTE !CONCAT(rva,!cnt)=UNIFORM(#tot_pop).				
		LEAVE !CONCAT(rva,!cnt).
		COMPUTE !CONCAT(rvb,!cnt)=!CONCAT(rva,!cnt)<t_weight.
	ELSE.
		COMPUTE !CONCAT(rvb,!cnt)=(!CONCAT(rva,!cnt)<t_weight) AND (!CONCAT(rva,!cnt)>lag(t_weight)).
	END IF.
!DOEND

COMPUTE flag=MAX(rvb1 TO !CONCAT('rvb',!draw)).
SAVE OUTFILE='draw file1.sav' /COMPRESSED.
SELECT IF (flag=1).
EXECUTE.
SAVE OUTFILE='draw file2.sav' /COMPRESSED.

VECTOR rvb=rvb1 TO !CONCAT(rvb,!draw).

* the same block may have been selected more than once but only the first occurence is kept.
COMPUTE done=0.
LOOP cnt=1 TO !draw.
	DO IF rvb(cnt)=1 AND done=0.
	XSAVE OUTFILE='draw file3.sav' /KEEP block cnt.
	COMPUTE done=1.
	END IF.
END LOOP.
EXECUTE.

GET
  FILE='draw file3.sav'.
SORT CASES BY cnt.
SELECT IF ($casenum<=!keep).
EXECUTE.

!ENDDEFINE.
*/////////////////////////// END OF MACRO//////////////////////////////.


* In this example, we want 120 blocks, 
we select 150 (with replacement) 
we delete blocks which were selected more than once
we keep the first 120 remaining blocks.

!SAMPLE draw=150 keep=120.
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