If the data ellipse (or, in this case, circle) is scaled so that its shadows (projections) on the axes each includes 68% of the data (that is of the marginal distribution of each variable), then the ellipse will include less than 68% of the data (i.e., of the joint distribution of the two variables). Conversely, to include 68% of the data in the ellipse, the shadows of the ellipse have to be larger.
Did I understand your point correctly?
John
At 09:40 PM 2/26/2003 +0100, [EMAIL PROTECTED] wrote:
Full_Name: Volker Franz Version: Version 1.6.2 (2003-01-10) OS: Debian Submission from: (NULL) (192.124.28.104)
Hi there,
it seems to me that data.ellipse of package "car" (Version 1.0-1) produces confidence interval's which are too big. To see this, do:
library(car) plot(c(-2,2),c(-2,2),pch=0) data.ellipse(rnorm(10000),rnorm(10000),levels=0.68,plot.points=F) abline(v=+1) abline(v=-1) abline(h=+1) abline(h=-1)
To my knowledge, this should result in a circle with radius 1. However, the circle is larger. It seems that the problem is due to an erroneous specification of the degrees of freedom and can be fixed with the following patch:
======================================================================
--- Ellipse.R Wed Feb 26 17:49:43 2003
+++ Ellipse.orig Thu Sep 19 18:20:41 2002
@@ -34,7 +34,7 @@
stop("x and y must be vectors of the same length")
if (plot.points & !add) plot(x, y, xlab=xlab, ylab=ylab, col=col, pch=pch,
las=las, ...)
if (plot.points & add) points(x, y, col=col, pch=pch, ...)
- dfn<-1
+ dfn<-2
dfd<-length(x)-1
if (robust) {
require(MASS)
======================================================================
Or --- am I totally on the wrong track here?
Best Volker
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____________________________ John Fox Department of Sociology McMaster University email: [EMAIL PROTECTED] web: http://www.socsci.mcmaster.ca/jfox
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