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#1
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Function for Logarithmic Trendline
I have been using the trendline - logarithmic type - on some charts. Although
the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#2
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To use the LINEST function for the coefficients of a log trendline see
Trendline coefficients http://www.tushar-mehta.com/excel/ti...efficients.htm -- Regards, Tushar Mehta www.tushar-mehta.com Excel, PowerPoint, and VBA add-ins, tutorials Custom MS Office productivity solutions In article , says... I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#3
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Thank you Tushar. I did look at your reference materials, and learned a lot.
However, I still do not see how to use LINEST to derive the coefficients of a logarithmic trendline. LINEST is for linear trends only, correct? Let me clarify my problem. On my chart I have a log trendline that displays as: y = 88.28Ln(x) - 38.613. It has a very good fit (r sqared of .97). I am looking for a function (or some method) of returning the coefficient values of 88.28 and -38.613 so that I can use them in subsequent calculations. Thanks, Art "Tushar Mehta" wrote: To use the LINEST function for the coefficients of a log trendline see Trendline coefficients http://www.tushar-mehta.com/excel/ti...efficients.htm -- Regards, Tushar Mehta www.tushar-mehta.com Excel, PowerPoint, and VBA add-ins, tutorials Custom MS Office productivity solutions In article , says... I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#4
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Art -
Using Excel's notation, the log trendline uses the equation Y = c*Ln(X)+b. c = SLOPE(Yrange,LN(Xrange)) b = INTERCEPT(Yrange,LN(Xrange)) Tushar's web site shows how to get the same results using =LINEST(y-range, LN(x-range)). The LINEST function (multiple X variables) and the Slope & Intercept functions (single X variable) may be used to fit linear combinations of variables, in this case, linear combinations of transformed variables. So they are not really limited to only linear trends. - Mike www.mikemiddleton.com "ArthurJ" wrote in message ... Thank you Tushar. I did look at your reference materials, and learned a lot. However, I still do not see how to use LINEST to derive the coefficients of a logarithmic trendline. LINEST is for linear trends only, correct? Let me clarify my problem. On my chart I have a log trendline that displays as: y = 88.28Ln(x) - 38.613. It has a very good fit (r sqared of .97). I am looking for a function (or some method) of returning the coefficient values of 88.28 and -38.613 so that I can use them in subsequent calculations. Thanks, Art "Tushar Mehta" wrote: To use the LINEST function for the coefficients of a log trendline see Trendline coefficients http://www.tushar-mehta.com/excel/ti...efficients.htm -- Regards, Tushar Mehta www.tushar-mehta.com Excel, PowerPoint, and VBA add-ins, tutorials Custom MS Office productivity solutions In article , says... I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#5
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Instead of LINEST, you could also use
=SLOPE(ydata,LN(xdata)) =INTERCEPT(ydata,LN(xdata)) for a and b. These formulas must also be array entered (Ctrl-Shift-Enter). Jerry ArthurJ wrote: I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#6
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Jerry (or Mike),
Is there an error in SLOPE or INTERCEPT contained in your response? They are identical ... ? I have only one independent x-variable. I need a step by step on this. Here is what I tried: I calculated ln(xdata) for each record. I then entered the SLOPE formula into one cell below the table exactly as written below. I then selected that cell and the adjoining cell to the right, pressed F2, then Ctrl-Shift-Enter. Both cells evaluate to the same number. Art "Jerry W. Lewis" wrote: Instead of LINEST, you could also use =SLOPE(ydata,LN(xdata)) =INTERCEPT(ydata,LN(xdata)) for a and b. These formulas must also be array entered (Ctrl-Shift-Enter). Jerry ArthurJ wrote: I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#7
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Mike,
I tried charting y-range and LN(x-range). It is not linear, but is still log shaped (concave downward), though not as much as the original chart of x-range,y-range. It doesn't make sense to me to calculate LINEST on this data which is not linear. I'm just not getting this yet. Art "Mike Middleton" wrote: "Tushar's web site shows how to get the same results using =LINEST(y-range, LN(x-range))". - Mike www.mikemiddleton.com "ArthurJ" wrote in message ... Thank you Tushar. I did look at your reference materials, and learned a lot. However, I still do not see how to use LINEST to derive the coefficients of a logarithmic trendline. LINEST is for linear trends only, correct? Let me clarify my problem. On my chart I have a log trendline that displays as: y = 88.28Ln(x) - 38.613. It has a very good fit (r sqared of .97). I am looking for a function (or some method) of returning the coefficient values of 88.28 and -38.613 so that I can use them in subsequent calculations. Thanks, Art "Tushar Mehta" wrote: To use the LINEST function for the coefficients of a log trendline see Trendline coefficients http://www.tushar-mehta.com/excel/ti...efficients.htm -- Regards, Tushar Mehta www.tushar-mehta.com Excel, PowerPoint, and VBA add-ins, tutorials Custom MS Office productivity solutions In article , says... I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#8
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Art -
There seem to be at least two things to consider. First, look at your data (e.g., an XY Scatter plot) and, based partly on what you plan to do with the results of your analysis, choose an appropriate functional form for a best fit. Second, use Excel to determine the parameters for the best-fit function. From your first post, I thought you were at the second step. Now, it seems you're back on the first step. So, you could post the data in the body of a message (if it's not too much) if you want us the "look at the data." And, describe the context of your analysis (what is it for?). - Mike "ArthurJ" wrote in message ... Mike, I tried charting y-range and LN(x-range). It is not linear, but is still log shaped (concave downward), though not as much as the original chart of x-range,y-range. It doesn't make sense to me to calculate LINEST on this data which is not linear. I'm just not getting this yet. Art "Mike Middleton" wrote: "Tushar's web site shows how to get the same results using =LINEST(y-range, LN(x-range))". - Mike www.mikemiddleton.com "ArthurJ" wrote in message ... Thank you Tushar. I did look at your reference materials, and learned a lot. However, I still do not see how to use LINEST to derive the coefficients of a logarithmic trendline. LINEST is for linear trends only, correct? Let me clarify my problem. On my chart I have a log trendline that displays as: y = 88.28Ln(x) - 38.613. It has a very good fit (r sqared of .97). I am looking for a function (or some method) of returning the coefficient values of 88.28 and -38.613 so that I can use them in subsequent calculations. Thanks, Art "Tushar Mehta" wrote: To use the LINEST function for the coefficients of a log trendline see Trendline coefficients http://www.tushar-mehta.com/excel/ti...efficients.htm -- Regards, Tushar Mehta www.tushar-mehta.com Excel, PowerPoint, and VBA add-ins, tutorials Custom MS Office productivity solutions In article , says... I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#9
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Art -
If your xdata is in A2:A8 and your ydata is in B2:B8, in some other cell type =SLOPE(B2:B8,LN(A2:A8)) and press Enter. In another cell, type =INTERCEPT(B2:B8,LN(A2:A8)) and press Enter. I don't think these functions need to be array-entered. - Mike www.mikemiddleton.com "ArthurJ" wrote in message ... Jerry (or Mike), Is there an error in SLOPE or INTERCEPT contained in your response? They are identical ... ? I have only one independent x-variable. I need a step by step on this. Here is what I tried: I calculated ln(xdata) for each record. I then entered the SLOPE formula into one cell below the table exactly as written below. I then selected that cell and the adjoining cell to the right, pressed F2, then Ctrl-Shift-Enter. Both cells evaluate to the same number. Art "Jerry W. Lewis" wrote: Instead of LINEST, you could also use =SLOPE(ydata,LN(xdata)) =INTERCEPT(ydata,LN(xdata)) for a and b. These formulas must also be array entered (Ctrl-Shift-Enter). Jerry ArthurJ wrote: I have been using the trendline - logarithmic type - on some charts. Although the equation in the form y = a*ln(x) + b can be made visible on the chart, I need a function that will return the a and b coefficients. I can't find such a function. I don't think logest is the same. Art |
#10
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Thanks for everyone who stuck with me on these questions. I now have
successfully implemented the suggested procedures contained in these replies, which included using linest, or slope and intercept to directly calculate the a and b coefficients of the natural log form. Art |
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