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Math Help - Calculating a value from a trendline.

  1. #1
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    Calculating a value from a trendline.

    After using the following numbers to create a calibration curve
    X
    133406
    418832
    844875

    y
    0.0408
    0.2221
    0.4341

    and adding a trendline the equation of the trendline is is
    y = 0.000000548500693x - 0.023105816308584

    How is it possible to use this to calculate the Y values for these X values?
    39231.3




    When I tried for the first number
    y = 0.000000548500693(39231.3) - 0.02311
    y = 0.0215183952372909 - 0.02311
    y = -0.00159160 which can't be possible
    when i'm guessing the answer should be around 0.0130

    Thank you in advance
    Last edited by lemwi; October 24th 2010 at 04:42 AM.
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  2. #2
    MHF Contributor Unknown008's Avatar
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    Your trendline is correct and your calculation is correct too.

    Why do you expect it to be 0.0130?
    Last edited by Unknown008; October 24th 2010 at 03:47 AM.
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  3. #3
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    From using the calibration numbers, it's around 1/3th of the first numbers
    133406 which gave a y value of 0.0408
    so 0.0400 / 3

    and the values are absorbency (x) against concentration (y) so for an absorbency value of 39231.3 to give a concentration of -0.00159160 isn't possible

    Would it mean that the 39231.3 value is just incorrect?
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  4. #4
    MHF Contributor Unknown008's Avatar
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    I think that you'll have instead to force your trendline to pass through (0, 0)

    I'm not sure about absorbency and concentration, but would concentration be 0 if the absorbency is 0? If yes, then you'll have to force it to (0, 0)

    And from there, you will use this new trendline to get the other values.
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  5. #5
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    I've got the trendline going though x which is showing a concentration of 0 giving an absorbency value of around 45000

    which gives the same trendline equation so the value of

    y = -0.00159160 for 39231.3 absorbency

    Which looks like the values for absorbency (39231.3) was just measured incorrectly?
    Last edited by lemwi; October 24th 2010 at 08:40 AM. Reason: trendline change
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  6. #6
    MHF Contributor Unknown008's Avatar
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    Well, if the trendline passes through (0, 0), then the equation will be in the form:

    y = mx

    only.
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