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Topic: correction to exp reg
Replies: 0

correction to exp reg
Posted: Jan 3, 1997 9:37 PM

On Jan 2 I posted a long discussion about fitting an exponential curve to
data. It included this paragraph:

......
Of course the exponential regression is not fit to the actual data, but
instead is a linear regression fit to (X,lnY) and then transformed back to
exponential form. This technique does not minimize the sum of squares of
the residuals, sum(Yi-A*B^Xi)^2 , but instead minimizes the sum of the
residuals of the log of the data, i.e. it finds a and b that minimizes
<<sum(lnYi-a-blnXi)^2>>. Then it calculates A=e^a and B=e^b for the equation
Y=A*B^X.
.........

There is a mistake above. The bit inside the << >> should read
sum(Yi-a-bXi)^2. That is, you don't take the ln of the Xi's when fitting an
exponential curve. Sorry about that. It doesn't affect the rest of the
discussion.

Doug

--
Doug Kuhlmann (508)-749-4242