Razzalinko Posted November 21, 2015 Share Posted November 21, 2015 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 Link to comment Share on other sites More sharing options...

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