Date:         Sat, 16 Jul 2011 15:14:38 -0700
Reply-To:     Daniel Nordlund <djnordlund@FRONTIER.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Daniel Nordlund <djnordlund@FRONTIER.COM>
Subject:      Re: the "standard" code generating a sample
In-Reply-To:  <4E220621.2070908@gmail.com>
Content-Type: text/plain; charset="UTF-8"

see comments below

> -----Original Message-----
> From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of Mark
> Miller
> Sent: Saturday, July 16, 2011 2:44 PM
> To: SAS-L@LISTSERV.UGA.EDU
> Subject: Re: the "standard" code generating a sample
>
> Max,
>
> Correction to my previous post
> The sample procedure is for SRS w Replacement
> due to use of POINT rather than sequential reads
> Not sure if Knuth covers this variant since
> I no longer have a copy to consult.
>
> ... Mark
>



No, the algorithm is for SRS without replacement.  The pointer value pickit runs consecutively from 1 to (potentially) totalobs.  It never repeats.

Hope this is helpful,

Dan

Daniel Nordlund
Bothell, WA USA







> On 7/16/2011 2:11 PM, bbser 2009 wrote:
> > I just realized I should have asked more specific questions about the
> > standard code.
> > So here we go:
> > 1. What's the purpose of the if expression?
> > 2. why do we need to this statement obsleft+(-1);
> > 3. How does this algorithm avoid picking up the same obs more than once?
> >
> > Thank you.
> >
> > Max
> > (Maaxx)
> >
> > -----Original Message-----
> > From: bbser 2009 [mailto:bbser2009@gmail.com]
> > Sent: July-16-11 4:47 PM
> > To: 'SAS-L@LISTSERV.UGA.EDU'
> > Subject: the "standard" code generating a sample
> >
> > Greetings!
> >
> > Basically speaking, the code below comes from the advanced certification
> > book.
> > This code are suppose to get a sample of size 10 from the pool.
> > But I do not understand the algorithm, especially the if expression.
> > Could you please explain the algorithm to me? Thank you very much!
> >
> > Max
> > (Maaxx)
> >
> > ======================
> >
> > data pool;
> > do pop=1 to 1000;
> > output;
> > end;
> > run;
> >
> > data sample(drop=obsleft samplesize);
> >          samplesize=10;
> >          obsleft=totobs;
> >          do while(samplesize>0);
> >                  pickit+1;
> >                  if ranuni(0)<samplesize/obsleft then
> >                          do;
> >                                  set pool point=pickit nobs=totobs;
> >                                  output;
> >                                  samplesize=samplesize-1;
> >                          end;
> >                          obsleft+(-1);
> >          end;
> >          stop;
> > run;
> >
> > proc print data=sample;
> > run;