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st: RE: i-1 in forvalues loop


From   Viktor Emonds <Viktor.Emonds@soc.kuleuven.be>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: i-1 in forvalues loop
Date   Tue, 18 Oct 2011 16:52:13 +0200

Let me try to explain again: I have a file with schools and I know the number of students in the target population. I have everything neatly sorted and for sampling, I just need to give each school a chance poportional to the number of students in the target year in that school on getting selected, determinate a random starting point to make my first pick and add a constant interval. 

In the loop with bal, I basically try to draw the 'winning numbers' by taking the constant (bal1) and adding the interval (3166.467), storing the new constant in bal2, bal3 ....bal15. The way I envisioned doing this was by running a loop for bal2-bal15, adding the interval to the value of the previous bal (the i-1th bal). Is there a way to do so?

Best,

Viktor

______________________________
I don't understand what you are trying to do. I comment only on obvious Stata problems. 
`i-1' would only work if "i-1" were the name of a local macro, but it can't be such a name, as minus signs are not allowed in Stata names. 
gen bal`i' = bal`=`i'-1' * 3166.467
would at first sight work as then Stata knows to evaluate the expression 
`i' - 1 
on the fly. 
Your code largely consists of putting constants into variables, which is legal but not especially good style. 
Note that 
gen lotto=sum(studentsj3)                                                  //sum of target population
produces a _cumulative_ sum: only in the last observation will this be the actual sum, as your comment implies. Whether the comment or the code is what you want only you can say. 
Nick

________________________________________
Van: Viktor Emonds
Verzonden: dinsdag 18 oktober 2011 15:55
Aan: statalist@hsphsun2.harvard.edu
Onderwerp: i-1 in forvalues loop

Hey,

I am trying to draw a sample with random start, fixed interval systematic sampling procedure in each explicit stratum. The data in each stratum are already sorted by all the implicit stratifiers with serpentine sorting for a variable of particular interest. Now I just need to do the actual sampling and tried to start by doing the following:

use ethnicframe31                                                              //the specific explicit stratum
gen lotto=sum(studentsj3)                                                  //sum of target population
egen bal1= mean(3166.4667*runiform())                             //random starting point
forvalues i=2/15 {                                                          //Draw ' lotto balls' by adding the fixed interval
gen bal`i'= bal`i-1'*3166.4667
}

gen winnaar=0                                                                 //Identify ' winning'  schools
forvalues i=1/15{
replace winnaar=1 if inrange(bal`i',lotto[_n-1],lotto)
}

Apparently, the `i-1'  in the first loop is not understood. What am I doing wrong here? Thanks in advance!
Best,
Viktor
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