Texas Instruments TI-82 Manual page 29

Press to move the trace cursor to the linear model
graph. The equation at the top of the screen tells you
which function you are tracing (in this case, Y
Use and/or
as possible and mark those points on your paper. Use a
ruler to connect the model points and draw the line.
¥ If you are copying the graph of a continuous curve, rather than a straight line, to your
paper, you need to trace as many points as necessary to see the shape of the curve and
mark the points on your paper. Connect the points with a smooth curve.
1.4.12 WHAT IS ÒBEST FITÓ?
b for the best fitting linear model y = ax + b, it is important to understand the method of
least-squares and the conditions necessary for its application if you intend to use this model.
You can explore the process of finding the line of best fit with program LSLINE. (Program
LSLINE is given in the TI-82/TI-83 Appendix.) For your investigations of the least-squares
process with this program, it is better to use data that is not perfectly linear and data for
which you do not know the best-fitting line.
Before using program LSLINE, clear the Y= list and enter your data in lists L
draw a scatter plot. Press
to help identify points on the graphics screen. Press
To activate program LSLINE, press
program, and press
pauses for you to view the screen.
¥ While the program is calculating, there is a small vertical line in the upper-right hand
corner of the graphics screen that is dashed and ÒmovingÓ. The program pauses several
times during execution. Whenever this happens, the small vertical line is ÒstillÓ and you
should press
The program next asks you to find the y-intercept and slope of some line you estimate will go
ÒthroughÓ the data. (You should not expect to guess the best fit line on your first try!) After
you enter a guess for the y-intercept and slope, your line is drawn and the errors are shown as
vertical line segments on the graph. (You may have to wait just a moment to see the vertical
line segments before again pressing
Next, the sum of squares of errors, SSE, is displayed for your line. Choose the TRY AGAIN?
option by pressing
change its slope to improve the fit to the data. After you enter another guess for the y -
intercept and/or slope, the process of viewing your line, the errors, and display of SSE is
repeated. If the new value of SSE is smaller than the SSE for your first guess, you have
improved the fit.
When it is felt that an SSE value close to the minimum value is found, you should press
at the TRY AGAIN? prompt. The program then overdraws the line of best fit on the graph for
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to locate values that are as ÒniceÓ
Even though your calculator easily computes the values a and
and reset Xscl and Yscl so that you can use the tick marks
WINDOW
PRGM
. The program first displays the scatter plot you constructed and
ENTER
to resume execution after you have looked at the screen.
ENTER
ENTER
. Decide whether you want to move the y-intercept of the line or
1 ENTER
.
1 )
to view the scatter plot.
GRAPH
followed by the number of the location of t h e
.)
A-29
Chapter 1
and L . Next,
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