Line of Best Fit
Using a Graphing Calculator
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So which line is the REAL "line-of-best-fit"? 

In the lesson on Line of Best Fit (without the graphing calculator), we discovered that finding the line-of-best-fit may lead to many "different" equations depending upon the points chosen to construct the line.

Simply stated, the graphing calculator has the capability of determining which line will actually represent the REAL line-of-best-fit.

The directions on this page were prepared using
the TI-83+/TI-84+ graphing calculator.

 Use of the graphing calculator will help you to better
understand the concept of line of best fit. 

Let's examine our fast food problem again with the use of the graphing calculator.

(This example and chart are from and were used with permission.)
Is there a relationship between the fat grams
and the total calories in fast food?

Sandwich Total Fat (g) Total Calories
Hamburger 9 260
Cheeseburger 13 320
Quarter Pounder 21 420
Quarter Pounder with Cheese 30 530
Big Mac 31 560
Arch Sandwich Special 31 550
Arch Special with Bacon 34 590
Crispy Chicken 25 500
Fish Fillet 28 560
Grilled Chicken 20 440
Grilled Chicken Light 5 300


Can we predict the number of total calories based upon the total fat grams?

1.  Enter the data in the calculator lists.  Place the data in L1 and L2.
STAT, #1Edit, type values into the lists


2.  Prepare a scatter plot of the data.
Set up for the scatterplot.
2nd StatPlot - choices shown at right.
Choose ZOOM #9 ZoomStat. Graph shown below.



3.  Have the calculator determine the line of best fit.
STAT → CALC #4 LinReg(ax+b)

Include the parameters L1, L2, Y1.
(Y1 comes from VARS → YVARS, #Function, Y1)

You now have the values of a and b needed to write the equation of the line of best fit.  See values at the right.
 y = 11.73128088x + 193.8521475


4.  Graph the line of best fit.
Simply hit GRAPH.

To get a predicted value within the window, hit TRACE, up arrow, and type the desired value.


Question:  Predict the total calories based upon 22 grams of fat.
ANS:  451.940 calories