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BrynPitt
USA
2 Posts |
 Posted - 06/15/2009 : 11:04:39 AM
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Origin Ver. and Service Release (Select Help-->About Origin):
Operating System:
Hello,
I just recently began using Origin. (I have Origin8.)
My question relates to finding values to fit functions. If I use Origin to fit a curve to a set of data, is there some way to determine the value of the function of the curve at certain points? (I.e.... if I have some function f(x), is there some way that I can find the value of f(x) for particular values of x?)
Also, is there some way I can make the equation of the fit curve show up on the graph without adding it manually. From reading the forums, I know this was possible using Origin7.5, but I don't see the option in Origin8.
Thanks,
Bryn |
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easwar
USA
1964 Posts |
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BrynPitt
USA
2 Posts |
Posted - 06/17/2009 : 6:17:34 PM
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Thank you for your help with the last problem. That was exactly what I wanted.
However, I have another problem now that is related.
When I fit a curve to a set of data, the equation that Origin gives doesn't seem to be the equation for the curve.
I have tried several different data sets (all with 4th Order Polynomial Fits), and each case the equation (in the form y=intercept+B1*x+B2*x^2+B3*x^3+B4*x^4)does not match the data or the outputs given when using "Find Y from X."
What is the equation that Origin gives in the table after I do a fit? And how do I find the equation for the curve that shows up on the graph after the fit?
I really need to know what equation (or other method) that Origin uses when doing "Find Y from X" for these fits.
Thanks again.
Thanks,
Bryn |
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easwar
USA
1964 Posts |
Posted - 06/17/2009 : 9:16:18 PM
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Hi Bryn,
With a 4th order polynomial fit, the equation used is indeed
y = Intercept + B1*x^1 + B2*x^2 + B3*x^3 + B4*x^4
as reported in the results sheet Notes table.
If you are however trying to reproduce/double check the numbers by taking the coefficients and plugging into a formula, the issue can be that the numbers displayed in the result sheet show fewer digits although internally the numbers are stored in full precision. With most fitting functions this is not an issue, but if you fit with high order polynomials, a few digits missing can make a big difference.
Do this: Go to Tools->Options menu, then click on Numeric Format tab, and change the Number of Digits drop-down to say 8. Then go back to the report sheet and look at the table (may need to refresh), you will see the coefficients for the polynomial are reported with more digits, and if you now recompute your numbers you will see they match. Again, internally Origin saves these coefficients in full precision, so the new Y values for new X values are indeed computed using the full precision coefficient values.
If you still have problems, please send your Origin project to tech support.
Easwar
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Values from fit functions