On Nov 30, 2008, at 10:23 PM, Dylan Beaudette wrote:
Hi, I am using the rcs() function in the Design library to model
non-linearity that is not well characterized by an otherwise
mechanistic function. I am able to make the model 'available' to
others through the excellent nomogram() function and the set of tables
that it can create. However, I would like to present the model in an
'expanded' format-- probably what rcspline.restate() or latex.Design()
produce on a model fit object.
Here is how the model was fit:
fit.ols <- ols( log(k) ~ (rcs(activity) * (log(conc) + sar)) +
(rcs(sand) * (log(conc) + sar)), data=sm.clean, x=TRUE, y=TRUE)
Here is how I am accessing the 'expanded' format of the model
structure:
options(digits=3)
latex(fit.ols, file='fit_rcs.tex')
The output contains several notation elements that I am not familiar
with:
1. x_{+} --> it seems that this represent a term that should be set
to 0, when x is 0?
It is set to zero when the term inside the cubic is less than zero.
See pages 20-21 of Harrell's book where the basis functions are
described and illustrated.
i.e. the entire expression −453(activity −
0.842)_{+}^{3} = 0 when
'activity' = 0 ??
.... whenever (activity − 0.842) < 0
2. the '!x' found in :
+log(conc) [ −0.0118sand + 9.58
! ×
!10−6 (sand − 11.6)
My guess is that this is 9.58 x 10^-6
− 0.000128(sand − 37.5)
+0.00045(sand − 47.2)
− 0.000350(sand − 51)
+ 1.86
! ×
!10−5 (sand − 69.8) ]
I don't see anything like that in Harrell's text and I am wondering if
a different character is getting rendering incorrectly. The only time
you see it is when the exponent is below -4.
.... what exactly does that mean?
An image version of the equation in question is attached.
Any input would be greatly appreciated!
Cheers,
Dylan
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and provide commented, minimal, self-contained, reproducible code.
