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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.
plot

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.)
list entry

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.

plot
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".

G&G Scatter Plot
x set
both sets
BurgerBoyDog

4. From MENU, choose #4 Window and #9 Zoom Data, to set the window for the plot.
Hit ENTER.
Zoom Data
scatter plot of data


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.


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