Re: [R] Fast version of Fisher's Exact Test

From: Steven McKinney <>
Date: Mon, 11 Apr 2011 10:06:51 -0700

Depends on how many other programs, and how large they are, and how much RAM you have on your machine. If I repeatedly run the example I used below, my R session shows 170MB of memory usage, not a huge amount relative to total memory, and not a huge amount even for 32 bit R. But if your system has 2 GB of RAM and 1.9 GB is consumed by other processes, then this example will cause swapping and speed will be reduced.

So figuring out a solution requires understanding what it is that is causing the slowdown - not enough RAM, other programs competing for CPU cycles...

You can try switching to 64 bit R but unless your 32 bit R is loading some large data objects, leaving little RAM, you won't see much of a difference. If you start R, and do rm(list = ls()) to ensure no big data objects are using up RAM, does the example below still take a long time?

You haven't mentioned what operating system you are using, how much RAM you have or what sessionInfo() reports on your machine. That information will help to figure this out.

Steven McKinney

From: Jim Silverton [] Sent: April 9, 2011 9:21 AM
To: Steven McKinney
Subject: Re: [R] Fast version of Fisher's Exact Test

I R 32 bit installed but my machine is 64 bit. Do I need to upgrade the R to 64 bit for it to run faster?

On Fri, Apr 8, 2011 at 6:44 PM, Steven McKinney <<>> wrote: Do you mean a test something such as this?

> fisher.test(matrix(c(502,498,490, 510), nrow = 2))

       Fisher's Exact Test for Count Data

data: matrix(c(502, 498, 490, 510), nrow = 2) p-value = 0.6228
alternative hypothesis: true odds ratio is not equal to 1 95 percent confidence interval:
 0.8770113 1.2550998
sample estimates:
odds ratio

This runs quickly on my machine.

> system.time(fisher.test(matrix(c(502,498,490, 510), nrow = 2)))
  user system elapsed
 0.008 0.001 0.010
> sessionInfo()

R version 2.12.2 (2011-02-25)
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)

[1] en_CA.UTF-8/en_CA.UTF-8/C/C/en_CA.UTF-8/en_CA.UTF-8

attached base packages:
[1] stats graphics grDevices utils datasets methods base

loaded via a namespace (and not attached): [1] tools_2.12.2

Can you provide an example that is running slowly for you?

Steven McKinney

From:<> [<>] On Behalf Of Jim Silverton [<>] Sent: April 8, 2011 9:43 AM
To:<> Subject: Re: [R] Fast version of Fisher's Exact Test

Is anyone aware of a fast way of doing fisher's exact test for a series of 2 x 2 tables in R? The fisher.test is really slow if n1=1000 and n2 = 1000.


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Received on Mon 11 Apr 2011 - 17:09:07 GMT

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