Regression by RMS method: Improving fit.

Hi all,

I'm looking for some help with a regression problem.

I have written a macro for Excel which fits a logarithmic function to a time series data set. The function I am using is:

Y’(x) = a*Ln(x + b) + c

Where a, b and c are constants, x is the julian date (days after 1st Jan 1900).

I input a guess for a, b and c the program than calculates the Sum of the Root Mean Squared residuals:

SSR = √[∑(Y(x) – Y’(x))^2/n]

Where Y(x) is the measured data at time x, and Y’(x) is the fitted value. n is the number of data points.

Excel then uses its Solver functionality to vary a, b and c to minimise SSR.

The problem is when I run this macro the fits are very poor. By eye the data looks vaguely curved, possibly logarithmic. However, the fitted functions look linear and do not closely represent the data.

I have speculated that the Solver is finding a local minimum of SSR, and not searching any further. In this case, could anyone suggest a way that I might improve my initial guesses of a, b and c to improve the chances of getting a closer fit?

Otherwise could anyone suggest an alternative (improved?) regression method?

I can show my code and/or a sample of the data if anyone is interested.

Thanks in advance (Happy)