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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(YiA*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(lnYiablnXi)^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(YiabXi)^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)7494242 Phillips Academy dkuhlman@idea.uml.edu Andover, MA 01810



