st: Unique Randomization within groups

[Jonah Rexer <jorexer@gmail.com>]

Hello,

I am doing a randomization among 3 groups (2 treatment and 1 control) for a
micro experiment.
Our sample is small, N=99 with the unit being a cluster (a group of 2-3
respondents).
I have merged clusters to assign each a random number. But instead of
stratifying, since our sample is so small, I have organized each cluster
into a group of 3 clusters (so 33 groups, each with 3 clusters) based on
our stratification variables.

What I want to do is assign a random integer, 1-3, within each group. I've
used the command:

set seed 4109

bysort cluster_group : gen random1=floor((3-1+1)*runiform()+1)

The problem is that this doesn't give me a unique number. Within each
group, i need one observation (cluster) to be 1, one to be 2, and one to be
3. This is why I used bysort, but it doesn't seem to be making any
difference. Is there any way to ensure that the number assigned is unique
within the group?
Thanks,

Jonah Rexer
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/