Chapter 12: Statistics
182
Notice the pattern of the residuals: a group of negative residuals, then a group of positive
residuals, and then another group of negative residuals.
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
… ~ ƒ
ã
A
ä
to select
A:PwrReg
from
the
STAT CALC
menu.
PwrReg
is pasted to the
home screen.
Press
y d † y e † † t a Í †
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.
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.