Operation Manual

Chapter 12: Statistics 188
The residual pattern indicates a curvature associated with this data set for which the linear model did
not account. The residual plot emphasizes a downward curvature, so a model that curves down with
the data would be more accurate. Perhaps a function such as square root would fit. Try a power
regression to fit a function of the form y = a … x
b
.
23. Press o to display the Y= editor.
Press ‘ to clear the linear regression equation
from
Y1. Press }Í to turn on Plot 1. Press
~Í to turn off Plot 2.
24. Press q
9 to select 9:ZoomStat from the ZOOM
menu. The window variables are adjusted
automatically, and the original scatter plot of time-
versus-length data (Plot 1) is displayed.
25. Press …~ƒã
Aä to select A:PwrReg from the
STAT CALC menu.
Press yd†ye††ta͆ to
highlight Calculate.
Note: You can also use the VARS Y-VARS FUNCTION
menu, ~1 to select Y1.
26. Press Í to calculate the power regression.
PwrReg is pasted to the home screen. Values for a
and b are displayed on the home screen. The power
regression equation is stored in
Y1. Residuals are
calculated and stored automatically in the list name
RESID.
27. Press s. The regression line and the scatter plot
are displayed.