R help - Mar 2012 - Which non-parametric regression would allow fitting this type of data? (example given).

Hi,

I'm wondering which function would allow fitting this type of data:

    tmp=rnorm(2000)
    X.1 = 5+tmp
    Y.1 = 5+ (5*tmp+rnorm(2000))
    tmp=rnorm(100)
    X.2 = 9+tmp
    Y.2 = 40+ (1.5*tmp+rnorm(100))
    X.3 = 7+ 0.5*runif(500)
    Y.3 = 15+20*runif(500)
    X = c(X.1,X.2,X.3)
    Y = c(Y.1,Y.2,Y.3)
   plot(X,Y)

The problem with loess is that distances for the "goodness of fit" are
calculated on the Y-axis. However, distances would need to be
calculated on the normals of the fitted curve. Is there a function
that provide this option?

A simple trick in that case consists in swapping X and Y, but I'm
wondering if there is a more general solution?

Thanks for your input,

Emmanuel

Thanks for the example.

Have you tried fitting a principal curve via either the princurve or
pcurve packages?  I think this might work for what you want, but no
guarantees.

Note that loess, splines, etc. are all fitting y|x, that is, a
nonparametric regression of y on x. That is not what you say you want,
so these approaches are unlikely to work.


-- Bert

On Sat, Mar 10, 2012 at 6:20 PM, Emmanuel Levy <emmanuel.levy at
gmail.com> wrote:
> Hi,
>
> I'm wondering which function would allow fitting this type of data:
>
> ? ?tmp=rnorm(2000)
> ? ?X.1 = 5+tmp
> ? ?Y.1 = 5+ (5*tmp+rnorm(2000))
> ? ?tmp=rnorm(100)
> ? ?X.2 = 9+tmp
> ? ?Y.2 = 40+ (1.5*tmp+rnorm(100))
> ? ?X.3 = 7+ 0.5*runif(500)
> ? ?Y.3 = 15+20*runif(500)
> ? ?X = c(X.1,X.2,X.3)
> ? ?Y = c(Y.1,Y.2,Y.3)
> ? plot(X,Y)
>
> The problem with loess is that distances for the "goodness of
fit" are
> calculated on the Y-axis. However, distances would need to be
> calculated on the normals of the fitted curve. Is there a function
> that provide this option?
>
> A simple trick in that case consists in swapping X and Y, but I'm
> wondering if there is a more general solution?
>
> Thanks for your input,
>
> Emmanuel
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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