[R] How many random numbers needed?

[Gavin Simpson]

On Sat, 2008-05-10 at 13:21 +0200, Birgit Lemcke wrote:
> Hello R-People!
> 
> I am running R 2.7.0 on a Power Book (Tiger). (I am still R and  
> statistics beginner)
> 
> Perhaps this is another stupid question of me, but I was wondering  
> how I know the needed random (set.seed) numbers, when running  
> randomForest (library randomForest) on a large dataset.

The seed is just a starting point for the RNG. You can draw as many
numbers as you like once the RNG has been seeded.

The ability to set the seed allows repeated runs of functions like
randomForest to provide the same results for each run. This is a basic
requirement of reproducible research.

require(randomForest)
set.seed(1)
mod1 <- randomForest(Species ~ ., data = iris)
mod2 <- randomForest(Species ~ ., data = iris)
set.seed(1)
mod3 <- randomForest(Species ~ ., data = iris)

all.equal(mod1, mod2)
all.equal(mod1, mod3)

You could put whatever (within reason - up to limits of an integer in R)
into the set.seed function, but the point is to provide the same number
in the seed if you want to make sure your results are reproducible.

HTH

G

> 
> Thanks in advance
> 
> Birgit
> 
> 
> 
> Birgit Lemcke
> Institut für Systematische Botanik
> Zollikerstrasse 107
> CH-8008 Zürich
> Switzerland
> Ph: +41 (0)44 634 8351
> birgit.lemcke at systbot.uzh.ch
> 
> 175 Jahre UZH
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