[R] studentized deleted residuals and NA's

[John Miyamoto]

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