Texas Instruments TI-89 Manual page 30

TI 89-30
These first differences are close to being constant. This information, together with the linear
pattern shown by the scatter plot, are a good indication that a linear function is likely to give a
good fit to the data.
1.5.7 FI NDI NG A LI NEAR FUNCTI ON TO MODEL DATA You will often have your TI-89
find the linear function that best fits a set of data. Other functions are fit using the same steps.
Access the data with
and choose
Matrix Editor]
Press
F5 [Calc].
(Calculation Type...)
Type in
[LinReg].
On the fourth line
press and choose where you want
►
to paste the linear function. Press
ENTER .
CAUTI ON: The best-fit function found by the calculator is also called a regression function.
The coefficients of the regression function never should be rounded! This is not a problem
because the calculator pastes the entire equation it finds into the
function is found if you follow the instructions given above.
Use to move between the various
▼
options in the
Calculate
you have entered all the information,
press
ENTER
and see that the function
F1 (Y )
=
has been pasted in the chosen location.
CAUTI ON: The number that is labeled corr on the
tion coefficient. This value and the one labeled R
are numbers that you will learn about in a statistics course. It is not appropriate
values in a calculus course.
Graphing the Line of Best Fit: After finding a best-fit equation, you should always draw the
graph of the function on a scatter plot to verify that the function provides a good fit to the data.
Press
F3 (GRAPH)
draw the function on the scatter plot of the data.
(As we suspected from looking at the scatter plot and the first
differences, this function provides a very good fit to the data.)
5
Unfortunately, there is no single number that can be used to tell whether one function better fits data than another. The
correlation coefficient only compares linear fits and should not be used to compare the fits of different types of functions.
For the statistical reasoning behind this statement, read the references in footnote 8 on page C-35.
APPS 6 [Data/
1 [Current].
On the first line
, press and
► 5
for and for .
c1 c2
x y
,
(Store RegEQ to...)
screen. When
to exit the screen. Press
or
F2 [Zoom] 9 [ZoomData]
list at the same time the
Y=
screen is called the correla-
STAT VARS
2
, which is the coefficient of determination,
to over-
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Chapter 1
5
to use these