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Math Help - Equation for Line Of Best Fit?

  1. #1
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    Question Equation for Line Of Best Fit?

    I have a chart out of which I made a graph, On Microsoft Excel (2011 for mac). I then added a Line of Best Fit.

    I need to answer questions about the graph, which include knowing the line of best fit. (Teacher said to use any statistical program to figure it out.....)

    How do I get the equation for Line of Best Fit/what is the equation?
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  2. #2
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    Re: Equation for Line Of Best Fit?

    If you are trying to find an equation for y as a function of x in the form y= mx+c given you have a sample size n for observations of x and y with means X and Y

    Find the sum of squares for y (SSyy)
    SS_{yy}= \sum_{i=1}^n (y_iy_i)-nY^2

    And the sum of squares for xy
    SS_{xy}= \sum_{i=1}^n (x_iy_i)-nXY

    Then
    m=\frac{SS_{xy}}{SS_{yy}}

    c= Y-m X
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  3. #3
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    Re: Equation for Line Of Best Fit?

    Appreciate your reply, but it looks like chinese to me .

    This is the "Data" I have been given:

    Annual Income Of Canadian Earners
    Year/ Lowest 20% Of Earners /Highest 20% Of Earners
    1977 12000 122500
    1978 11800 126400
    1979 13100 121100
    1980 13700 123200
    1981 14600 123200
    1982 14300 122300
    1983 12800 127600
    1984 13900 126300
    1985 14800 130700
    1986 15100 134300
    1987 15800 137000
    1988 15700 142800
    1989 16800 147800
    1990 15500 143100
    1991 14300 139100
    1992 13900 137400
    1993 14100 136700
    1994 14100 136300
    1995 14300 139900
    1996 13000 141900
    1997 13200 146500
    1998 13600 153900
    1999 13500 162300
    2000 14300 169600
    2001 14700 169400
    2002 14700 167800
    2003 14800 166700
    2004 14200 170600
    2005 14300 168500
    2006 14700 168000
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  4. #4
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    Re: Equation for Line Of Best Fit?

    You say you put this into Excel and "added a Line of Best Fit". I interpret that to mean that you used an Excel command to plot that line. Does Excel not have a way of giving the equation? If not you could still find two points on that line and use those to find the equation of the line. Shakarri gives the standard formulas for the coefficients of the line but Excel should already have done that for you.
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  5. #5
    Senior Member sakonpure6's Avatar
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    Re: Equation for Line Of Best Fit?

    if you go to your data points that excel plotted, select one and right click then go to "Add trendline" and at the bottom of the window that pops up is a box that says "Display equation on chart" select it and voila!
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  6. #6
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    Re: Equation for Line Of Best Fit?

    ^Thanks! Thought there HAD TO be a function in excel that did it!
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