-----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