Polynomial Graph

[Razzalinko]

Hi Guys,

I am a sports science PhD student having some problems using a polynomial equation to estimate what y will equal at x.

When I fit a 3rd order polynomial regression line, I recieve the following equation:

y = 2E-06x^3 + .0007x^2 - .424

Which as I understand when substituting 196 for x is the same as

y = 2x10^(-6)*196^3+.0007*196+.0984*196-.424

Based on the trendline I expect the datapoint at 196 to be approximately 4.26 to 4.28, however I get 7.03

I'm not sure what I'm doing wrong and any help would be greatly appreciated, the excel file is attached.

The data has been collected at 15 s intervals during exercise, complete exhaustion occured at 196 s and I would therefore like to estimate what, based on the curve, the data point at the fictitious 196 s mark would likely have been. It is not that important for this data but I'd like to apply the correct formula to get extra data for 10 s into the interval.

Thank you!

Damon

 

Data.xlsx