[R] Randomly interleaving data frames while preserving order

[Kevin E. Thorpe]

Hello.

I am trying to simulate recruitment in a randomized trial. Suppose I 
have three streams (strata) of patients represented by these data frames.

df1 <- data.frame(strat=rep(1,10),id=1:10,pid=1001:1010)
df2 <- data.frame(strat=rep(2,10),id=1:10,pid=2001:2010)
df3 <- data.frame(strat=rep(3,10),id=1:10,pid=3001:3010)

What I need to do is construct a data frame with all of these combined 
where the order of selection from one of the three data frames is 
randomized but once a stratum is selected patients are selected 
sequentially from that data frame.

To see what I'm looking to achieve, suppose the first five subjects were 
to come, in order, from strata (data frames) 1, 2, 1, 3 and 2. The 
expected result should look like this:

rbind(df1[1,],df2[1,],df1[2,],df3[1,],df2[2,])
    strat id  pid
1      1  1 1001
2      2  1 2001
21     1  2 1002
4      3  1 3001
22     2  2 2002

I hope what I'm trying to accomplish makes sense. Maybe I'm missing 
something obvious, but I really have no idea at the moment how to 
achieve this elegantly. Since I need to simulate many trial recruitments 
it needs to be general and compact.

I appreciate any advice.

Kevin

-- 
Kevin E. Thorpe
Head of Biostatistics,  Applied Health Research Centre (AHRC)
Li Ka Shing Knowledge Institute of St. Michael's
Assistant Professor, Dalla Lana School of Public Health
University of Toronto
email: kevin.thorpe at utoronto.ca  Tel: 416.864.5776  Fax: 416.864.3016