[R] R nls results different from those of Excel ??
rolf.turner at xtra.co.nz
Tue Feb 19 07:14:48 CET 2013
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.)
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
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