I'm going through my notes from SAS Programming 3 and I'm having trouble making sense of the example code. The goal here is to select random observations without duplicates.   data subset(drop=ObsLeft SampSize); SampSize=10; ObsLeft=TotObs; do while(SampSize>0 and ObsLeft>0); PickIt+1; if ranuni(0)<SampSize/ObsLeft then do; ObsPicked=PickIt; set orion.orderfact point=PickIt nobs=TotObs; output; SampSize=SampSize-1; end; ObsLeft=ObsLeft-1; end; stop; run; I think I understand why they use  ranuni(0)<SampSize/ObsLeft  to determine whether an observation is selected.  It compares a random number to the ratio of remaining samples to remaining observations.  In order to give each observation the same chance of being selected?   But my question is whether this is actually giving each sample the same probability of being selected.  If the ratio changes with each iteration then doesn't each sample have a different probability of being selected?  Im not sure that it matters but the definition given for "simple random sampling" was "equal probability" of being selected.  Couldn't you just use a fixed ratio of initial sampsize/totobs? My other question is what is preventing this from reaching the end of the dataset before outputting the required number of samples?  Theoretically, the random number could never match the condition for output before it gets to the end.   The next page in my notes is about PROC SURVEYSELECT so i'm assuming this example is just academic, but those things were bugging me. ... View more