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Math Help - Extrapolation formulae

  1. #1
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    Extrapolation formulae

    Hi,

    Can you help me out?

    I've data as follows:
    X Y
    8 5.93
    10 5.87
    12 5.84

    I want to extapolate the graph and find out the value where this graph cuts the Y-axis i.e. values for 0,2,4 & 6.
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  2. #2
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    Quote Originally Posted by kshrini View Post
    Hi,

    Can you help me out?

    I've data as follows:
    X Y
    8 5.93
    10 5.87
    12 5.84

    I want to extapolate the graph and find out the value where this graph cuts the Y-axis i.e. values for 0,2,4 & 6.
    Put the data into Excel (or some other spread sheet) and plot it as a scatter plot. Select the line and right click and select add trend line, select linear for the type of trend.

    Now right click on the trend line and select format trend line, click on display equation, also setup forcast to forcast backwards by 8 units.

    Now read all you need off of the plot.

    The attachment shows what you should have

    RonL
    Attached Thumbnails Attached Thumbnails Extrapolation formulae-gash.jpg  
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  3. #3
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    Extrapolation formulae

    Hi,

    Thanks for reply. But I want to program it in Viusal Basic. So I want the formula.

    Thanks once again.
    Shrinivas
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  4. #4
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    Quote Originally Posted by kshrini View Post
    Hi,

    Thanks for reply. But I want to program it in Viusal Basic. So I want the formula.

    Thanks once again.
    Shrinivas
    The Excel help system entry for trend line will give you the equations
    for the trend line calculation.

    RonL
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  5. #5
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    Extrapolation formulae

    Hi,

    Thanks for reply. My data is changing, as I'm acquiring it from instrument. So extrapolation will be done at the time of data acquisition.

    If you have the formula then I can manage it programatically.

    Shrinivas
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  6. #6
    Grand Panjandrum
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    Quote Originally Posted by kshrini View Post
    Hi,

    Thanks for reply. My data is changing, as I'm acquiring it from instrument. So extrapolation will be done at the time of data acquisition.

    If you have the formula then I can manage it programatically.

    Shrinivas
    See here for the equations.

    Summarising these:

    Put

    s_{x,y}=sum(x_i y_i)

    s_{x}=sum(x_i)

    s_{y}=sum(y_i)

    s_{x,x}=sum(x_i^2)

    s_{y,y}=sum(y_i^2)

    Then:

    m=\frac{n s_{x,y}-s_x s_y}{n s_{x,x}-s_x^2},

    where n is the number of data points.

    and:

    b=\frac{s_y-m s_x}{n}.

    Then the regression line is y=m x+b and y=b when x=0.

    RonL
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  7. #7
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    Extrapolation formulae

    Hi,

    Thanks a lot. This is perfectly OK if data has linear relation. But if the data is exponential then how to extrapolate it?

    Shrinivas
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