[R] the plotting position of theoretical quantile for qqnorm
Martin Maechler
maechler at stat.math.ethz.ch
Thu Feb 8 09:57:48 CET 2007
>>>>> "LizF" == fengfeng <fengfeng99 at gmail.com>
>>>>> on Wed, 7 Feb 2007 23:08:31 -0600 writes:
LizF> Hello,
LizF> I have a doubt about the plotting position of the theoretical quantile for
LizF> the qqnorm
LizF> command in R.
LizF> Let F be the theoretical distribution of Y, we observed a sample of size n,
LizF> y1,y2, ...,
LizF> yn. We then sort it and comspare these empirical quantiles to the expected
LizF> ones
LizF> from F. For the plotting poition, there are several options:
LizF> 1. i/(n+1)
LizF> 2. (i-.375)/(n+.25)
LizF> 3. (i- .3175)/ (n + .365)
LizF> etc.
yes, particularly "etc" ;-)
LizF> Which one is "qqnorm" used?
It's right in front of you if you read help(qqnorm) carefully :
>> See Also:
>>
>> 'ppoints', used by 'qqnorm' to generate approximations to expected
>> order statistics for a normal distribution.
>>
So it uses ppoints() and help(ppoints) tells you what's going
on: The formula used depends on (n <= 10) but see that help page.
LizF> Thx a lot!
You're welcome,
Martin Maechler, ETH Zurich
LizF> Liz
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