[R] R nls results different from those of Excel ??

[Rolf Turner]

To paraphrase Bill Venables (see fortune(217)):

     Simple.  Excel must be broken.  Have you reported it to them?

(The difference in this case is that it is probable that Excel *is* broken.
It usually is.)

     cheers,

         Rolf Turner

On 02/19/2013 05:49 PM, David Gwenzi wrote:
> Hi all
>
> I have a set of data whose scatter plot shows a very nice power
> relationship. My problem is when I fit a Power Trend Line in an Excel
> spreadsheet, I get the model y= 44.23x^2.06 with an R square value of 0.72.
> Now, if I input the same data into R and use
> model< -nls(y~ a*x^b , trace=TRUE, data= my_data, start = c(a=40, b=2)) I
> get a solution with a = 246.29 and b = 1.51. I have tried several starting
> values and this what I always get. I was expecting to get a value of a
> close to 44 and that of b close to 2. Why are these values of a and b
> so different from those Excel gave me. Also the R square value for the nls
> model is as low as 0.41. What have I done wrong here? Please help. Thanks
> in advance
>
> David
>
> 	[[alternative HTML version deleted]]
>
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