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fitting a hyperbole

4 messages · stephen sefick, Peter Dalgaard, Rolf Turner

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I have got a data set that is Gross Primary Productivity ~ Total
Suspended Solids it is a hyperbola just like:
plot(1/c(1:1000))

how do I model this relationship so that I can get all of the neat
things that lm gives residuals etc. etc. so that I can see if my
eyeball model stands up.  Thanks for any help, pointers, or good
things to read.
#
I am not sure if I am exaggerating or not read title as hyperbola
On Sat, Sep 20, 2008 at 2:20 PM, stephen sefick <ssefick at gmail.com> wrote:

  
    
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stephen sefick wrote:
Well, it depends on the exact model you want to fit and the error 
characteristics.

There's a straightforward linear model in the transformed x:
lm(y ~ I(1/x))

but there are also transformed models like

lm(1/y ~ x)

or

lm(log(y) ~ log(x))

but of course, y, 1/y, and log(y) can't all be homoscedastic normal 
variates. Going beyond the linearized models, you can use nls(), as in

nls(y~ a/(x-b), start=c(a=1,b=0))

(which is linear for 1/y, but assumes that y rather than 1/y has 
constant variance.)

  
    
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On 21/09/2008, at 10:38 AM, Peter Dalgaard wrote:

            
Nicely expressed.  Succinct, clear, to the point, comprehensive.  I  
wish I'd said that!

(And that's not hyperbole. :-) )

So much more helpful than some postings I've seen recently to the  
effect of ``Go away
and read a book on this topic.''

	cheers,

		Rolf

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