[R] Iterative sampling with restrictions

[David L Carlson]

If I understand you, this is just random assignment over 6 groups: 1/2, 1/3,
2/1, 2/3, 3/1, 3/2. 

> groups <- c("1/2", "1/3", "2/1", "2/3", "3/1", "3/2")
> assign <- sample(rep(groups, 10))
> table(assign)
assign
1/2 1/3 2/1 2/3 3/1 3/2 
 10  10  10  10  10  10

----------------------------------------------
David L Carlson
Associate Professor of Anthropology
Texas A&M University
College Station, TX 77843-4352

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Daniel Nordlund
> Sent: Tuesday, August 21, 2012 2:58 AM
> To: r-help at r-project.org
> Subject: Re: [R] Iterative sampling with restrictions
> 
> > -----Original Message-----
> > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org]
> > On Behalf Of David A. Kim
> > Sent: Monday, August 20, 2012 9:17 PM
> > To: r-help at r-project.org
> > Subject: [R] Iterative sampling with restrictions
> >
> > Hi all,
> >
> > I'm working on a seemingly trivial problem that I can't figure out
> how
> > to implement in R.  I'd be most grateful for any help here.
> >
> > I want to do the following: first, randomly assign each of n units to
> > one of g groups of size n/g. Then, randomly re-assign each of the n
> > units to a different group (i.e., same as the first step, but the
> unit
> > can't be assigned to a group to which it's already belonged).  Then
> > repeat this step until each unit has at some point been assigned to
> > every group.
> >
> > More concretely, say I have 60 units and 3 groups into which to
> divide
> > the units.  I could first do something like:
> >
> > group1<-sample(1:60,20)
> > group2<-sample(setdiff(1:60,group1),20)
> > group3<-sample(setdiff(1:60,c(group1,group2)),20)
> >
> > But then how to randomly re-assign group membership such that all
> > units are assured a different group assignment in the second "wave"
> of
> > grouping?  Just narrowing the sampling pool to those units that
> > weren't previously assigned to a given group won't work (consider the
> > case where groups 1 and 2 swap units: in the second wave, there would
> > be no units to assign to group 3 as all the remaining units had
> > already been in group 3 in the first wave).
> >
> > Most grateful for any assistance,
> >
> > David
> >
> 
> David,
> 
> I would collect the sample waves into a data.frame.  I am sure someone
> will be able to help you with a more general and/or efficient solution,
> but to get you started I have provided one possible solution to your 60
> unit 3 wave example
> 
> #create data.frame with IDs
> df <- data.frame(id=1:60)
> 
> #create first sample wave
> df$wave1 <- sample(rep(1:3,20))
> 
> #reorder df and create second wave sample
> df <- df[order(df$wave1),]
> df$wave2 <-  c(sample(rep(c(2,3),10)),
>                sample(rep(c(1,3),10)),sample(rep(c(1,2),10)))
> 
> #now use set diff to create 3rd wave
> for(i in 1:60) df[i,'wave3'] <- unlist(setdiff(1:3,df[i,2:3]))
> 
> df
> 
> 
> Hope this is helpful,
> 
> Dan
> 
> Daniel Nordlund
> Bothell, WA USA
> 
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