Have you plotted the data and the lines to see how they compare?  (see
fortune(193)).

Is there error around the line in the data?  The nls function is known to
not work well when there is no error around the line.   Also check and make
sure that the 2 methods are fitting the same model.

You might consider taking the log of both sides of the function to turn it
into a linear function and using lm to fit the logs.


On Mon, Feb 18, 2013 at 9:49 PM, David Gwenzi <dgwenzi@gmail.com> 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
>
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>
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>



-- 
Gregory (Greg) L. Snow Ph.D.
538280@gmail.com

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