# Fitting curves to curved decay rate and comparison thereof in minitab

Hi Folks,

Can anyone explain how to do the following in Minitab:

Want to fit a curve to a curved decay rate and then compare curves with something like analysis of covarience to see if them are the same or significantly different. Thought the curve would be an exponential decay but appears to be more cubic decay.

Lots of points on this one because I haven't a clue either about the answer or the question ;0)

Thanks,

Jes
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Commented:
I don't have or know MINITAB.  But MS Excel has lots of curve fitting and statistical functions, including covariance (COVAR).

Can you post the data so we can cee it??
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Author Commented:
Hi Folks,

@d-glitch: Sorry, I can't post the data as it's for my wifes Phd (and she'd probably kick my butt if I did) and also for some reason minitab is the tool of choice so other application options aren't, um, options ;0)

Thanks for trying.

Later,

Jes
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Commented:
I am not familiar with minitab, but can make some comments on the curve fitting problem which may help you define the problem better.
I presume that by cubic decay you mean a curve like
y = A/(x^3)
and by exponential decay you mean a curve like
z = Ce^(Bx) and that you have a bunch of data points to which you want to fit the above curves and then compare the fit.

By the proper (usually least squares) method you can get values of A, B, and C. You will find that both curves look very similar (if the domain of x is reasonably small). If the theory suggests one or the other take that one.. If there is no theory, take the one with the least sum of the squares of the difference. If there is no theory perhaps some other curve would fit even better. In fact there is guarenteed to be a better curve.
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Commented:

I understand that you can't post preliminary, propriatary data.

But could you post similar dummy data, or at least describe you real data?

For example:     20 sets of data.  Each set consists of 50 measurements of [population, concentration, temperature...] versus time.
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Commented:
Minitab also has  online tutorials that may cover the topics you need:       http://www.minitab.com/resources/tutorials/
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Author Commented:
Hi Folks,

Well, my wife being a smartie (she'd have to be to understand all of the above) finally figured it out herself.  But you guys tried to help, so have some points.

Later,

Jes
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