Scatter Plots

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

 In many real-life situations, scatter plots follow patterns that are approximately linear.  If y tends to increase as x increases, then the paired data are said to be a positive correlation.  If y tends to decrease as x increases, the paired data are said to be a negative correlation.  If the points show no linear pattern, the paired data are said to have relatively no correlation.

Setting up a scatter plot

Example:  Is there a relationship between the fat grams and the total calories in fast food? (refer to the data below)

 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
1.  Enter the Fat grams into column A of the spreadsheet. Label the column "fat".  Enter the Calories into column B and label the column "cal". Be sure you have the same number of entries in both columns.

(See Lists and Spreadsheets for entering data in lists.)

Once the data is entered, there are two ways to obtain a scatter plot.

METHOD 1: (Data & Statistics)

2.  Graph the scatter plot. From HOME, choose #5 Data and Statistics. Hit ENTER.
Using the Nav Pad, arrow to the bottom of the screen and choose the x-variable to be "fat". Then move to the left side of the screen and choose the y-variable to be "cal". You will now see your scatter plot.

 METHOD 2: (Graphs & Geometry) 2.  From HOME, choose #2 Graphs&Geometry. From MENU, choose #3 Graph Type and #4 Scatter Plot, as seen at the right. Hit ENTER. 3. At the bottom of the screen, highlight the box naming the set to be used for the x-values, click, and choose "fat". Use your TAB key to highlight box if needed. Do the same for the y set and choose "cal". 4. From MENU, choose #4 Window and #9 Zoom Data, to set the window for the plot. Hit ENTER.

In both methods, visual examination shows a positive correlation between the total grams of fat and the total calories. (The graph resembles a straight line rising to the right.)

Let's move on to "line of best fit" and see how the calculator can find the best equation to represent this data.