Just as a rule of thumb, extrapolating the results of a polynomial fit (of higher order than a quadratic) much further than the "delta x" of one data point beyond your data is not a good idea. That means if your data are spaced a month apart, you don't want to extrapolate more than a month past your data set. The reason is that a polynomial of the form: 
y = ax + bx**2 + cx**3 + dx**4 and so on

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will be dominated by the high order terms outside the range of your data, and rapidly go to infinity (positive or negative). These kinds of functions suck at extrapolation.

There are other functions that are better suited to extrapolation. Without knowing how your data are *supposed* to behave, I'd be hesitant to recommend any offhand.

Update: typo

In reply to Re: Polynomial Trendline / Extrapolation by dwm042
in thread Polynomial Trendline / Extrapolation by lwicks

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