**From:** John Miyamoto (*jmiyamot@u.washington.edu*)

**Date:** Tue 06 Apr 2004 - 05:24:50 EST

**Next message:**Steve Roberts: "[R] 2 lme questions"**Previous message:**WilDscOp: "[R] Selecting Best Regression Equation : leaps() in R and stepwise() in S+"

Message-id: <Pine.A41.4.58.0404051219110.1052698@mead11.u.washington.edu>

Dear R-Help,

I am using the studres function from the MASS package to compute

studentized deleted residuals in a oneway anova. I'm having trouble

interpreting the results in situations where a factor level has only one

observation. Sometimes studres yields an NaN and sometimes it produces a

numeric value for cases where a factor level has only one observation. I

would think it should always produce NaN for this case. The following

example illustrates this point.

library(MASS)

Data <- c(3,6,3,4,1,5,6,2)

Group <- factor(c(1,1,2,3,3,4,5,5))

lm.out <- lm(Data ~ Group)

data.frame(Data, Group,

hat=lm.influence(lm.out)$hat,

std.res=stdres(lm.out),

stu.del.res=studres(lm.out))

OUTPUT:

Data Group hat std.res stu.del.res

1 3 1 0.5 -8.9113279e-01 -8.4852814e-01

2 6 1 0.5 8.9113279e-01 8.4852814e-01

3 3 2 1.0 Inf NaN

4 4 3 0.5 8.9113279e-01 8.4852814e-01

5 1 3 0.5 -8.9113279e-01 -8.4852814e-01

6 5 4 1.0 -1.5208938e-08 -1.2418046e-08

7 6 5 0.5 1.1881771e+00 1.3333333e+00

8 2 5 0.5 -1.1881771e+00 -1.3333333e+00

Warning messages:

1: NaNs produced in: sqrt((n - p - sr^2)/(n - p - 1))

2: NaNs produced in: sqrt((n - p - sr^2)/(n - p - 1))

Groups 2 and 4 both have only one observation. Both the standardized

residual (std.res) and studentized deleted residual (stu.del.res) are NaN

for Group = 2 but both have a value for Group = 4. I would think that

std.res and stu.del.res should be NaN for Group 2 and for Group 4 because

1 - h_ii = 0 for the corresponding cases. Can someone tell me why the

std.res and stu.del.res are different for Group = 2 and Group = 4?

I am running R 1.8.1 on a Windows XP machine.

John

--------------------------------------------------------------------

John Miyamoto, Dept. of Psychology, Box 351525

University of Washington, Seattle, WA 98195-1525

Phone 206-543-0805, Fax 206-685-3157, Email jmiyamot@u.washington.edu

Homepage http://faculty.washington.edu/jmiyamot/

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**Next message:**Steve Roberts: "[R] 2 lme questions"**Previous message:**WilDscOp: "[R] Selecting Best Regression Equation : leaps() in R and stepwise() in S+"

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