[R] strange thing with sd

[Hanke, Alex]

Just so you don't feel that you are alone....
I get the same response as you use R1.8.0 on an XP operating system.
Alex

-----Original Message-----
From: Andreas Pauling [mailto:pauling at giub.unibe.ch] 
Sent: March 29, 2004 10:02 AM
To: r-help at stat.math.ethz.ch
Subject: [R] strange thing with sd


Dear R people

I came across a strange thing: 


sd(rep(0.01,      15))  #0
sd(rep(0.001,     15))  #4.489023e-19
sd(rep(0.00001,   15))  #0
sd(rep(0.00000001,15))  #1.712427e-24

sd(rep(0.01,      13))  #1.805557e-18
sd(rep(0.001,     13))  #4.513894e-19
sd(rep(0.00001,   13))  #0
sd(rep(0.00000001,13))  #0

sd(rep(5.01,	  15))  #0
sd(rep(5.001,	  15))  #4.489023e-19
sd(rep(5.00001,   15))  #1.838704e-15
sd(rep(5.00000001,15))  #9.19352e-16

sd(rep(5.01,	  13))  #9.244454e-16
sd(rep(5.001,	  13))  #9.244454e-16
sd(rep(5.00001,   13))  #1.848891e-15
sd(rep(5.00000001,13))  #0

Why gives sd sometimes zero and sometimes values close to zero
and why does it depend on the value and on how many times it is
repeated?
Shouldn't it give always zero?
Is there a way to control this?

I use R Version 1.8.1 under UNIX.

Thanks for any suggestions!!

Andreas

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