[R] interpretation of RCS 'coefs' and 'knots'
debeaudette at ucdavis.edu
Sat Oct 24 00:46:48 CEST 2009
I have fit a series of ols() models, by group, in this manner:
l <- ols(y ~ rcs(x, 4))
... where the series of 'x' values in each group is the same, however knots
are not always identical between groups. The result is a table of 'coefs'
derived from the ols objects, by group:
group Intercept top top' top''
1 6.864 0.01 2.241 -2.65
2 6.836 0.047 -0.556 0.606
3 5.877 -0.019 0.084 -0.175
4 6.021 -0.003 0.121 -0.128
5 7.164 0.014 0.031 -0.096
I would like to describe groups of relationships, based on the coefficients,
however I am not sure if they are directly comparable. In addition, I would
like to regress these coefs on another set of variables, with the aim of
predicting a series of RCS coefficients along external gradients. In essence,
I am hoping to use RCS coefficients to summarize y ~ rcs(x), in a way that
can then me modeled like this: [y ~ rcs(x)] ~ z.
Is this interpretation of RCS coefficients even possible? If not, would
forcing knot locations make it a possibility? Or, would modeling both knots
and RCS coefs with external variables lead to sensible predictions?
Soil Resource Laboratory
University of California at Davis
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