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Thread: Algebra, Excel, Statistics, and fan curves...Question.

  1. 12-02-2007, 09:53 PM #1
    h3rk
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    Algebra, Excel, Statistics, and fan curves...Question.

    It's been a while since I formally learned any algebra, and I've never really spent much time with statistics.

    Example PDF.

    What I want to do, is take a set of about 5 points (pressure,flow) and create a 2nd order polynomial from them, just the same way that excel does it with the trendline function on a chart. I could just cut and paste the shown equation and use it to complete more points,but..

    The application I have for this needs to use the equation as a function: ax^2+bx+c; where a, b, and c are stored as constants, after 'calibrated' by intrepreting a minimum number of points.

    How does excel 'interpret' the points for a poly? Is it a proprietary combination of LINEST and LOGEST? Where's a good place to find out?

    How many points would be a good minimum for accuracy?

    How would I do it mathematically, without using excel?

    Any points in the right direction (other sites would be fine too) would be much appreciated.
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  2. 12-02-2007, 10:31 PM #2
    lbridges
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    For a polynomial curve fit I believe you will be looking at a least squares problem, solving matrix equations, etc. This really is a computational suited task - I think it would take a long, long time by hand. I'm sure the math methods are out there if you plug a few your words into google.

    there might be newer methods - I've been out of school for a lot of years.
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  3. 12-02-2007, 10:45 PM #3
    h3rk
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    After researching a bit, it seems to me you're right about the least squares polynomial fit, thanks for the pointer
    It made an excellent search term for me.
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  4. 12-02-2007, 11:28 PM #4
    h3rk
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    I found what I'm looking for, and for anyone interested:

    SPLINE    -
    library of C++ routines, using double precision arithmetic, for constructing and evaluating spline functions.
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