I'd like to use these data to derive an equation using Excel.
300 13 310 12.6 320 12.2 330 11.8 340 11.4 350 11 360 10.8 370 10.6 380 10.4
As x goes up, y goes down. Seems straightforward. But when I do a polynomial regression on these data, even though the trendline matches the data pretty well, the equation it generates doesn't work. The equation is
0.0096x2 - 0.4181x + 13.341 When I plug in x values to that equation, the numbers go up! So something is pretty wrong here.
- place both number series in excel
- select the second set (13, 12.6 ...)
- plot a line graph
- set the first set as the x axis labels
- select Series1 and add a polynomial (2) trendline, display equation, display R-squared
That produces the equation above, with an R^2 value of .9955. But when I use that equation, it doesn't produce those outputs for those inputs.
Clearly I'm doing something wrong.
Edit: or is it excel? Here's the graph of that equation (above):
Clearly that is not trending downward for the range 300-390.
Here's the real equation that fits this data:
Quadratic Fit: y=a+bx+cx^2
a = 4.53E+01 b = -1.66E-01 c = 1.95E-04
Thank you CurveExpert 1.4.