expand.grid

Thursday, July 6, 2006

Here’s a simple trick for creating experimental designs in R: use the function expand.grid.

A simple example is:

  treatments <- LETTERS[1:4]
  levels <- 1:3
  experiment <- data.frame(expand.grid(treatment=treatments, level=levels))

which produces:

   treatment level
1          A     1
2          B     1
3          C     1
4          D     1
5          A     2
6          B     2
7          C     2
8          D     2
9          A     3
10         B     3
11         C     3
12         D     3

Now, if you want to randomize your experimental treatments, try:

  experiment[sample(dim(experiment)[1]), ]

sample randomly chooses numbers from a vector the same length as the experiment data frame without replacement. The square brackets then use this random sample to subsample from the experiment data frame.

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