Texas Instruments TI-85 - Manual (Page 28)

B-28 TI-85/TI-86 Graphing Calculators
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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(x)= list and enter your data in lists L 1 and L 2 .
Next, draw a scatter plot. Reset xScl and yScl so that you can use the tick marks to help
identify points on the graphics screen. Press GRAPH to view the scatter plot.
To activate program LSLINE , press PRGM F1 ( NAMES ) followed by the F -key under t h e
program, and press ENTER . (Since your calculator ÒloadsÓ each program in memory when you
run it, longer programs such as this one take a moment before anything appears on t h e
screen.) The program first displays the scatter plot you constructed and 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. You
should press ENTER to resume execution.
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 ENTER .)
Next, the sum of squares of errors, SSE, is displayed for your line. Choose the TRY AGAIN ?
option by pressing 1 ENTER . Decide whether you want to move the y-intercept of the line or
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 2
at the TRY AGAIN ? prompt. The program then overdraws the line of best fit on the graph for
comparison with your last attempt and shows the errors for the line of best fit. The
coefficients a and b of the best-fitting linear model y = ax + b are then displayed along with
the minimum SSE . Use program LSLINE to explore the method of least squares to find t h e
line of best fit.
1.4.13 NAMING DATA LISTS (optional) You may or may not want to use the additional
features given below for data entered on your calculator. You can name data (either input,
output, or both) and store it in the calculator memory for later recall.
For instance, suppose you wanted to call the following list L 1 by another name :
1984 1985 1987 1990 1992
First, enter the data in the list L 1 .
Return to the home screen.
(You can view any list from the home screen by typing
its name and pressing ENTER . )