[R] strange thing with sd

[Peter Flom]

I tried your sample code, and a few other variations, and got 0 for all
of them, running R 1.8.1 on a Windows machine

Peter

Peter L. Flom, PhD
Assistant Director, Statistics and Data Analysis Core
Center for Drug Use and HIV Research
National Development and Research Institutes
71 W. 23rd St
www.peterflom.com
New York, NY 10010
(212) 845-4485 (voice)
(917) 438-0894 (fax)



>>> Andreas Pauling <pauling at giub.unibe.ch> 3/29/2004 9:01:40 AM >>>
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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