Excel definitely does not use nonlinear least squares fitting for power curve
fitting. It uses linear LS fitting of the logs of x and y. There should be no
surprise in the OP's observation.
---------------------------------------------------------------------------
Jeff Newmiller The ..... ..... Go Live...
DCN:<jdnew...@dcn.davis.ca.us> Basics: ##.#. ##.#. Live Go...
Live: OO#.. Dead: OO#.. Playing
Research Engineer (Solar/Batteries O.O#. #.O#. with
/Software/Embedded Controllers) .OO#. .OO#. rocks...1k
---------------------------------------------------------------------------
Sent from my phone. Please excuse my brevity.
Greg Snow <538...@gmail.com> wrote:
>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 <dgwe...@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
>>
>> [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help@r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
