[R] plotting additive ns components

[David Winsemius]

On Jan 27, 2010, at 9:09 PM, GlenB wrote:

>
>
> I have an additive model of the following form :
>
> zmdlfit <- lm(z~ns(x,df=6)+ns(y,df=6))
>
> I can get the fitted values and plot them against z easily enough,  
> but I
> also want to both obtain and plot the two additive components (the  
> estimates
> of the two additive terms on the RHS)
>
> I've been looking at manuals and searching on the internet and  
> searching the
> archives, but I'm apparently incompetent because I can't locate it -  
> how do
> I plot just the x and y splines (against x and y)?
>
> I've read the help on predict.lm, and on predict.ns (/predict.bs)  
> but it
> only shows how to get the new columns for new values of x; I could  
> multiply
> those by the coefficients of the spline fit, and I could also do it by
> holding each variable fixed while the other varies in predict (which  
> is
> right up to an additive constant), but it seems like there would  
> have to be
> a more straightforward that way I am missing. It looks like gam and  
> mgcv do
> it for you, but can I do it with just lm and ns?
>

I don't know about lm and ns but in either the Design package or its  
successor, rms, you could have obtained the expression by using the  
Function function. You might look at the methods Harrell concocted if  
you don't want to switch over and do it the easy way.

-- 
David.


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David Winsemius, MD
Heritage Laboratories
West Hartford, CT