Randomly interleaving data frames while preserving order

Tue, Mar 31, 2015 11:06 AM

-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Kevin
E. Thorpe
Sent: Tuesday, March 31, 2015 10:53 AM
To: Duncan Murdoch
Cc: R Help Mailing List
Subject: Re: [R] Randomly interleaving data frames while preserving
order

On 03/31/2015 01:44 PM, Duncan Murdoch wrote:

On 31/03/2015 1:05 PM, Kevin E. Thorpe wrote:

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.

How about something like this:

# Permute an ordered vector of selections:
sel <- sample(c(rep(1, nrow(df1)), rep(2, nrow(df2)), rep(3,

nrow(df3))))

# Create an empty dataframe to hold the results
df <- data.frame(strat=NA, id=NA, pid=NA)[rep(1, length(sel)),]

# Put the original dataframes into the appropriate slots:
df[sel == 1,] <- df1
df[sel == 2,] <- df2
df[sel == 3,] <- df3

# Clean up the rownames
rownames(df) <- NULL

Duncan Murdoch

Thanks Duncan.

Once you see the solution it is indeed obvious.

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

Another option would be to stack your strata and then sample from the combined data frame, something like this:

sample_size <- 10
population <- rbind(df1,df2,df3)
sim.sample <- pop[sample(nrow(pop),sample_size, replace=FALSE),]

Hope this is helpful,

Dan

Daniel J. Nordlund, PhD
Research and Data Analysis Division
Services & Enterprise Support Administration
Washington State Department of Social and Health Services

Randomly interleaving data frames while preserving order

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