On Aug 26, 2010, at 7:10 PM, Peter Dunn wrote:
Hi all
I was playing with termplot(), and came across what appears to be an
inconsistency.
It would appreciate if someone could enlighten me:
# First, generate some data:
y <- rnorm(100)
x <- runif(length(y),1,2)
# Now find the log of x:
logx <- log(x)
# Now fit two models that are exactly the same, but specified
differently:
m1 <- lm(y~log(x)) # Using log in the call
m2 <- lm(y~logx) # Using the variable logx
# The two termplots() are different:
par(mfrow=c(1,2))
termplot(m1)
termplot(m2)
I see two models that are identical, but produce different
termplot()s. In both cases, the independent variable is log(x), but
is just specified differently.
If this is intended, what is the logic? Or am I missing something?
I don't understand your complaint, The two termplots are both on the
scale of the x-variable offered via the data argument (or in this case
the formula argument) . In one case you offered it a transformed
value; in the other you offered it an untransformed value and did the
transformation within the lm function. termplot() "knew" or was able
to discern what was going on in the second instance and had no way of
"knowing" that a transformation had been done in the first. Where is
the puzzle? (Forgive the anthropomophisation.)
str(m1)
.
.
.
$ terms :Classes 'terms', 'formula' length 3 y ~ log(x)
.. ..- attr(*, "variables")= language list(y, log(x))
.. ..- attr(*, "factors")= int [1:2, 1] 0 1
.. .. ..- attr(*, "dimnames")=List of 2
.. .. .. ..$ : chr [1:2] "y" "log(x)"
.. .. .. ..$ : chr "log(x)"
.. ..- attr(*, "term.labels")= chr "log(x)"
--
David.
Thanks.
P.
sessionInfo()
R version 2.11.1 (2010-05-31)
x86_64-apple-darwin9.8.0
locale:
[1] en_AU.UTF-8/en_AU.UTF-8/C/C/en_AU.UTF-8/en_AU.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
David Winsemius, MD
West Hartford, CT
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