> -----Original Message-----
> From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org]
> On Behalf Of David A. Kim
> Sent: Monday, August 20, 2012 9:17 PM
> To: r-help@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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