1. Scatter Plots and Line of Best Fit
Algebra I - Unit 5
Scatter Plots and Line of Best Fit

2. What is a scatter plot?
A scatter plot is a graph used to determine whether there is a relationship between paired data.

3. 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
550
Arch Special with Bacon 34
590
Crispy Chicken 25
500
Fish Fillet 28
Grilled Chicken 20
440
Grilled Chicken Light 5
300

4. Press Stat and choose 1: Edit… Enter the x-data in L1
Press Stat and choose 1: Edit… Enter the x-data in L1. Enter the y-data in L2.
Press Stat Plot (for scatterplot ) and choose 1: Plot 1. Be sure the plot is ON, the scatter plot is highlighted, and that the correct lists are used. Choose any of the three marks.
Press Zoom and choose 9: ZoomStat to see the scatter plot.

5. What is a line of best fit?
A line of best fit (or “trend line”) is a straight line that best represents the data on a scatter plot.
This line may pass through . . .
some of the points,
none of the points,
or all of the points.

6. Press Stat, over to Calc and choose 4:LinReg(ax+b). Press Enter
Press Stat, over to Calc and choose 4:LinReg(ax+b). Press Enter. You now have the values of a and b needed to write the equation of the line of best fit.
Press Y= and enter the equation.
Press Graph to see the line on the scatter plot.

7. Making predictions
Once you have an equation for the line of best fit, you can use that equation to make predictions.
If you are looking for values that fall within the plotted values, you are interpolating.
If you are looking for values that fall outside the plotted values, you are extrapolating.
CAUTION: The further away from the plotted values you go, the less reliable is your prediction.

8. How many calories?
If a sandwich has 15 grams of fat, how many calories would you expect it to have?
This is an interpolation!
If a sandwich has 50 grams of fat, how many calories would you expect it to have?
This is an extrapolation!