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Polynomial Trendline Issues

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    Polynomial Trendline Issues

    Hello,

    For my excel project, I'm plotting data from a profilometer program.

    What I'm trying to do is get a trendline for my given data, put the equation for the trendline and plug in the X values, and compare the given data to the trendline inputted data.

    The problem is that for whatever reason, the trendline equation is incorrect.
    I plotted the data I have with the trendline on top of it, and then plotted X vs. the Trendline data; it comes out completely different.

    I've attached my file, can anyone see what the hell is going on?

    [Note: there is a lot of data so please be patient as it does freeze up computers for a minute or two
    Also since my file was too big, I saved it as a binary excel file (.xlsb) but since you don't allow that on your site, I renamed it to a .xls file; It will say it may be corrupted but opens fine]
    Attached Files Attached Files

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    re: Polynomial Trendline Issues

    Doesn't open for me.

    You can get the trendline coefficients using LINEST, and then reconstruct the data using SERIESSUM. If you can sort that out, you're golden. If not, figure out how to post a representative sample of your data.
    Entia non sunt multiplicanda sine necessitate

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    Re: Polynomial Trendline Issues

    I'm not trying to make a straight line, but a polynomial line [right now I'm using a 6th order polynomial equation because it yields the highest R^2 value]

    I've linked the file; its bigger than the allowable upload size for this website

    http://www.filehosting.org/file/deta.../Sample_A.xlsx

    [This may look sketchy but I tested it a few times and the file is virus free]

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    Re: Polynomial Trendline Issues

    How about putting it on a file sharing site that doesn't want my email address to download?

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    Re: Polynomial Trendline Issues


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    Re: Polynomial Trendline Issues

    I decimated your data down to about 475 rows, and then used LINEST to get the coefficients of the fit. (Because of the large x values, there's some scaling used to get these.)

    You can change the value of B3 from 1 to 6 to see different fits. When the value is 6, the coefficients are the same as what the trendline is on the chart shows (and the Fit plot is hidden behind the trendline because they are coincident).

    The LitArr function in the LINEST formula is a convenience. It could be replaced with a ROW(INDIRECT("1:"...)) construct.
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    Re: Polynomial Trendline Issues

    Is everything supposed to have the "#VALUE" sign around ? I'm confused as to what you did; I need all of the data points in order to conduct analysis

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    Re: Polynomial Trendline Issues

    Is everything supposed to have the "#VALUE" sign around ?
    No, of course not. Do Tools > Add-Ins, tick Analysis Toolpak
    I need all of the data points in order to conduct analysis
    You can paste all your values in the x-y range. I didn't need them all to create an example ...

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