Texas Instruments TI-85 User Manual

Page 28

B-28

TI-85/TI-86 Graphing Calculators

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

and

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

(

NAMES

) followed by the

-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

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

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

by another name

1984

1985

1987

1990

1992

First, enter the data in the list

Return to the home screen.

(You can view any list from the home screen by typing

its name and pressing

ENTER

. )