Excel trendline accuracy

Posted by Rook on Super User See other posts from Super User or by Rook
Published on 2010-09-29T12:56:16Z Indexed on 2012/06/09 4:43 UTC
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microsoft-excel

|

charts

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trendlines

This is a problem I have every once in a while, and it annoys me tremendously, beacuse I have always to recheck every trendline I get.

An example:

r       L
(mm)    
30,00   97,0  
60,00   103,2  
90,00   106,0  
110,00  101,0  
125,00  88,0  
140,00  62,0  
148,00  36,7  
152,50  17,0

Upon drawing a trendline (using 3rd order polynomial regression type) with r on the x axis, and L on the y one, Excel will give the formula

y = -0,0002x³ + 0,0341x² - 1,8979x + 128,73

with R² = 0,994. If I interpolate values using that formula for the same values of r as the ones the formula was derived from, I get

r   y  
(mm)      
30,00   97,083  
60,00   94,416  
90,00   88,329  
110,00  66,371  
125,00  33,68  
140,00  -17,416  
148,00  -53,5912  
152,50  -76,97725

which are quite different?

Why does this happen? What is the reason for it?

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