R nls results different from those of Excel ??

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]]

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 at 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]]

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.

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Jeff Newmiller <jdnewmil <at> dcn.davis.ca.us> writes:

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.

May I be allowed to say that the general comments on MS Excel may be
alright,
in this special case they are not.  The Excel Solver -- which is made
by an
external company, not MS -- has a good reputation for being fast and
accurate.
And it indeed solves least-squares and nonlinear problems better than
some of
the solvers available in R.
There is a professional version of this solver, not available from
Microsoft,
that could be called excellent. We, and this includes me, should not be
too 
arrogant towards the outside, non-R world, the 'barbarians' as the
ancient 
Greeks called it.

Hans Werner

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Jeff Newmiller                        The     .....       .....  Go

Live...

DCN:<jdnewmil <at> dcn.davis.ca.us>   Basics: ##.#.       ##.#.  Live

Go...

                                      Live:   OO#.. Dead: OO#.. 

Playing

Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
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rocks...1k

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Sent from my phone. Please excuse my brevity.