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#1
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How do I improve Accuracy of trendlines?
I am taking known data (4 points) and creating a trendling. I am selecting
the option to show the formula and R*R value, which is 1.0. I am charting this equation and comparing it to the data used to create the formula, and to the chart used to derive the formula. They are very different. Can I correct this situation? If so, how? I have checked to make sure that I typed in the correct formula. Where do I go from here? |
#2
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How do I improve Accuracy of trendlines?
Tom21 -
(1) Use more significant digits for your calculations. (A) Select the trendline equation on the chart, and press the Increase Decimal button repeatedly, or (B) use worksheet formulas, like INTERCEPT, SLOPE, or LINEST for your calculations. (2) Use an XY (Scatter) chart type. If you use a Line chart type, Excel uses X values of 1,2,3,... for the calculations. (3) Give us more information about the data, the chart type, and the trendline Trend/Regression type you are using. - Mike www.mikemiddleton.com "Tom21" wrote in message ... I am taking known data (4 points) and creating a trendling. I am selecting the option to show the formula and R*R value, which is 1.0. I am charting this equation and comparing it to the data used to create the formula, and to the chart used to derive the formula. They are very different. Can I correct this situation? If so, how? I have checked to make sure that I typed in the correct formula. Where do I go from here? |
#3
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How do I improve Accuracy of trendlines?
Right click on the equation and format to display scientific notation with 14
decimal places. Also, make sure that your chart is an "XY (Scatter)" chart and not a "Line" chart. The "Line" chart is very misleadingly named; it has nothing to do with whether you want a line or not--rather it considers the x-data (if given at all) to be category labels instead of numbers. Why it offers to fit a trendline when it does not believe the x-data to be numeric is a mystery; but when asked to do so, it assumes that the x-data are 1,2,3,... instead of the values that you may have supplied. As a result, trendlines on "Line" charts are usually worse than meaningless. Jerry "Tom21" wrote: I am taking known data (4 points) and creating a trendling. I am selecting the option to show the formula and R*R value, which is 1.0. I am charting this equation and comparing it to the data used to create the formula, and to the chart used to derive the formula. They are very different. Can I correct this situation? If so, how? I have checked to make sure that I typed in the correct formula. Where do I go from here? |
#4
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How do I improve Accuracy of trendlines?
Mike,
Thank you for your help. I was already using an XY Scatter chart type. I increased the number of decimal places to 14. The equation got a little messier, and the R*R value went down ever so slightly, but the created function now matches the data. "Mike Middleton" wrote: Tom21 - (1) Use more significant digits for your calculations. (A) Select the trendline equation on the chart, and press the Increase Decimal button repeatedly, or (B) use worksheet formulas, like INTERCEPT, SLOPE, or LINEST for your calculations. (2) Use an XY (Scatter) chart type. If you use a Line chart type, Excel uses X values of 1,2,3,... for the calculations. (3) Give us more information about the data, the chart type, and the trendline Trend/Regression type you are using. - Mike www.mikemiddleton.com "Tom21" wrote in message ... I am taking known data (4 points) and creating a trendling. I am selecting the option to show the formula and R*R value, which is 1.0. I am charting this equation and comparing it to the data used to create the formula, and to the chart used to derive the formula. They are very different. Can I correct this situation? If so, how? I have checked to make sure that I typed in the correct formula. Where do I go from here? |
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