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#1
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How to get a polinom trend eq. for data series not starting at 0?
I'm trying to fit a 3rd degree trend line to a data series with independent
values starting from -8 to 10. The 3rd degree equation that I get keeps assuming that the first independent data point is zero, and so, the y-intercept of the equation is not the value for -8. These are the data points, m Freq (cm-1) -8 2107 -7 2112 -6 2116 -5 2120 -4 2124 -3 2128 -2 2132 -1 2136 0 2139 1 2147 2 2151 3 2155 4 2158 5 2162 6 2166 7 2169 8 2173 9 2176 10 2180 And so, the y-intercept at x=0 should be close to 2139 instead of the 2104 value that I get. How to make excel recognize that the first data point is not zero? Please help, Juan |
#2
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I get y=-0.0041x^3-0.0121x^2+4.3565x+2141.2 for the trendline equation
With LINEST (see my site www.stfx.ca/people/bliengme/ExcelTips) I get the same: -0.00412 -0.01211 4.356495 2141.183 What makes you think the 'real' value of the intercept is 2139? Please explain more what you mean by "The 3rd degree equation that I get keeps assuming that the first independent data point is zero" By the way, you are working with a "polynomial" best wishes -- Bernard V Liengme www.stfx.ca/people/bliengme remove caps from email "juannrobless" wrote in message ... I'm trying to fit a 3rd degree trend line to a data series with independent values starting from -8 to 10. The 3rd degree equation that I get keeps assuming that the first independent data point is zero, and so, the y-intercept of the equation is not the value for -8. These are the data points, m Freq (cm-1) -8 2107 -7 2112 -6 2116 -5 2120 -4 2124 -3 2128 -2 2132 -1 2136 0 2139 1 2147 2 2151 3 2155 4 2158 5 2162 6 2166 7 2169 8 2173 9 2176 10 2180 And so, the y-intercept at x=0 should be close to 2139 instead of the 2104 value that I get. How to make excel recognize that the first data point is not zero? Please help, Juan |
#3
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Juan -
You are using the Line chart type (which uses values 1,2,3,... for fitting a trendline). Instead, use the XY (Scatter) chart type (which will use -8,-7,...). - Mike www.mikemiddleton.com "juannrobless" wrote in message ... I'm trying to fit a 3rd degree trend line to a data series with independent values starting from -8 to 10. The 3rd degree equation that I get keeps assuming that the first independent data point is zero, and so, the y-intercept of the equation is not the value for -8. These are the data points, m Freq (cm-1) -8 2107 -7 2112 -6 2116 -5 2120 -4 2124 -3 2128 -2 2132 -1 2136 0 2139 1 2147 2 2151 3 2155 4 2158 5 2162 6 2166 7 2169 8 2173 9 2176 10 2180 And so, the y-intercept at x=0 should be close to 2139 instead of the 2104 value that I get. How to make excel recognize that the first data point is not zero? Please help, Juan |
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