On Thu, 13 Apr 2006, Spencer Graves wrote:
> Do you know of a reference that discusses alternative choices for
> plotting positions for a normal probability plot? The documentation for
> qqnorm says it calls ppoints, which returns qnorm((1:m-a)/(m+1-2*a))
> with "a" = ifelse(n<=10, 3/8, 1/2)? The help pages for qqnorm and
> ppoints just refer to Becker, Chambers and Wilks (1988) The New S
> Language (Wadsworth & Brooks/Cole), and I couldn't find any discussion
> of this.
>
> I seem to recall that this was discussed in 1960 or earlier in a
> paper by Anscombe, but I can't find a reference and I wonder if someone
> might suggest something else. I've been asked to comment on specialized
> software that allows the user to select "a" = +/-0.5, 0, 0.3, and 0.3175
> (but not 0.375 = 3/8, curiously).
>
> I'd also be interested in any examples of real data sets where the
> choice of "a" actually made a difference. When I've had so few data
> points that the choice for "a" might make a difference, a normal
> probability plot was not very informative, anyway, and I get more
> information from a simple dot plot. If your experience is different,
> I'd like to know.
>
> Thanks,
> Spencer Graves
>
I suspect that what you are looking for is this paper:
Hyndman, Rob J. and Fan, Yanan (1996)
Sample quantiles in statistical packages
The American Statistician, 50, 361-365
which discusses different definitions of sample quantiles. See also the
documentation for quantile.
David Scott
_________________________________________________________________
David Scott Department of Statistics, Tamaki Campus
The University of Auckland, PB 92019
Auckland NEW ZEALAND
Phone: +64 9 373 7599 ext 86830 Fax: +64 9 373 7000
Email: [EMAIL PROTECTED]
Graduate Officer, Department of Statistics
ASC/NZSA 2006: "Statistical Connections"
The joint conference of the Statistical Society of Australia Inc.
and the New Zealand Statistical Association, July 3--6, 2006
in Auckland. Go to:
http://www.statsnz2006.com/
______________________________________________
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
