Trend formula completely wrong

[Beleriand_K]

When I enter the following data into Excel and generates af Forecast Analysis with a Polynomial Trendline (Order 2) Excel shows an almost perfect trendline in the diagram. But when I choose Display equation on chart, it comes up with a Trend formula that has nothing to do with reality.

This is Excels Trendline formula:

Y = -0,0044x^2+0,6225x+13,408

If I put three of the numbers above into this formula, the results are this:

80 => 35,048 (real result in table: 13,88)
100 => 31,658 (real result in table: 24,53)
120 => 24,748 (real result in table: 31,63)

Apart from being very different from the results in the table, the Y-values gets smaller in the Trend formula, while they get bigger in the input table. So there seems to something fundamentally wrong with the equation.

I have searched the internet for an explanation, but without succes. The only answer I have found is, that I should use more decimals, but I have tried to extend the number of decimals to 30, and it doesn't help.

I suppose Excel is working as it should, so it must be me doing something wrong. Can anyone figure out, what it is? Because I surely can't.




[TABLE="width: 500"]
<tbody>[TR]
[TD]80[/TD]
[TD]13,88[/TD]
[/TR]
[TR]
[TD]81[/TD]
[TD]14,54[/TD]
[/TR]
[TR]
[TD]82[/TD]
[TD]15,18[/TD]
[/TR]
[TR]
[TD]83[/TD]
[TD]15,80[/TD]
[/TR]
[TR]
[TD]84[/TD]
[TD]16,41[/TD]
[/TR]
[TR]
[TD]85[/TD]
[TD]17,01[/TD]
[/TR]
[TR]
[TD]86[/TD]
[TD]17,59[/TD]
[/TR]
[TR]
[TD]87[/TD]
[TD]18,16[/TD]
[/TR]
[TR]
[TD]88[/TD]
[TD]18,72[/TD]
[/TR]
[TR]
[TD]89[/TD]
[TD]19,29[/TD]
[/TR]
[TR]
[TD]90[/TD]
[TD]19,79[/TD]
[/TR]
[TR]
[TD]91[/TD]
[TD]20,31[/TD]
[/TR]
[TR]
[TD]92[/TD]
[TD]20,82[/TD]
[/TR]
[TR]
[TD]93[/TD]
[TD]21,32[/TD]
[/TR]
[TR]
[TD]94[/TD]
[TD]21,81[/TD]
[/TR]
[TR]
[TD]95[/TD]
[TD]22,29[/TD]
[/TR]
[TR]
[TD]96[/TD]
[TD]22,75[/TD]
[/TR]
[TR]
[TD]97[/TD]
[TD]23,21[/TD]
[/TR]
[TR]
[TD]98[/TD]
[TD]23,66[/TD]
[/TR]
[TR]
[TD]99[/TD]
[TD]24,10[/TD]
[/TR]
[TR]
[TD]100[/TD]
[TD]24,53[/TD]
[/TR]
[TR]
[TD]101[/TD]
[TD]24,95[/TD]
[/TR]
[TR]
[TD]102[/TD]
[TD]25,36[/TD]
[/TR]
[TR]
[TD]103[/TD]
[TD]25,77[/TD]
[/TR]
[TR]
[TD]104[/TD]
[TD]26,17[/TD]
[/TR]
[TR]
[TD]105[/TD]
[TD]26,56[/TD]
[/TR]
[TR]
[TD]106[/TD]
[TD]26,94[/TD]
[/TR]
[TR]
[TD]107[/TD]
[TD]27,31[/TD]
[/TR]
[TR]
[TD]108[/TD]
[TD]24,68[/TD]
[/TR]
[TR]
[TD]109[/TD]
[TD]28,04[/TD]
[/TR]
[TR]
[TD]110[/TD]
[TD]28,40[/TD]
[/TR]
[TR]
[TD]111[/TD]
[TD]28,75[/TD]
[/TR]
[TR]
[TD]112[/TD]
[TD]29,09[/TD]
[/TR]
[TR]
[TD]113[/TD]
[TD]29,43[/TD]
[/TR]
[TR]
[TD]114[/TD]
[TD]29,76[/TD]
[/TR]
[TR]
[TD]115[/TD]
[TD]30,08[/TD]
[/TR]
[TR]
[TD]116[/TD]
[TD]30,40[/TD]
[/TR]
[TR]
[TD]117[/TD]
[TD]30,72[/TD]
[/TR]
[TR]
[TD]118[/TD]
[TD]31,03[/TD]
[/TR]
[TR]
[TD]119[/TD]
[TD]31,33[/TD]
[/TR]
[TR]
[TD]120[/TD]
[TD]31,63[/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]

 

[shg]

You're welcome.