Before we take up the discussion of linear regression and correlation, we need to examine a way to display the relation between two variables x and y. The most common and easiest way is a scatter plot. The following example illustrates a scatter plot.

Example 12.5

In Europe and Asia, m-commerce is popular. M-commerce users have special mobile phones that work like electronic wallets, as well as provide phone and Internet services. Users can do everything from paying for parking to buying a TV set or soda from a machine to banking to checking sports scores on the internet. For the years 2000 through 2004, was there a relationship between the year and the number of m-commerce users? Construct a scatter plot. Let x = the year and let y = the number of m-commerce users, in millions.

Table showing the number of m-commerce users (in millions) by year
xx (year) yy (no. of users)
2000 0.5
2002 20.0
2003 33.0
2004 47.0
Table 12.1
This is a scatter plot for the data provided. The x-axis represents the year and the y-axis represents the number of m-commerce users in millions. There are four points plotted, at (2000, 0.5), (2002, 20.0), (2003, 33.0), (2004, 47.0).
Figure 12.5 Using the x- and y-coordinates in the table, we plot the points on a graph to create the scatter plot showing the number of m-commerce users (in millions) by year. We can now use this to scatter plot to look for trends in the data.

Using the TI-83, 83+, 84, 84+ Calculator

To create a scatter plot:
  1. Enter your x data into list L1 and your y data into list L2.
  2. Press 2nd STATPLOT ENTER to use Plot 1. On the input screen for PLOT 1, highlight On and press ENTER. (Make sure the other plots are OFF.)
  3. For TYPE, highlight the first icon, which is the scatter plot, then press ENTER.
  4. For Xlist, enter L1 ENTER; for Ylist, enter L2 ENTER.
  5. For Mark, it does not matter which symbol you highlight, but the square is the easiest to see. Press ENTER.
  6. Make sure there are no other equations that could be plotted. Press Y = and clear out any equations.
  7. Press the ZOOM key and then the number 9 (for menu item ZoomStat); the calculator will fit the window to the data. You can press WINDOW to see the scaling of the axes.
Try It 12.5

Amelia plays basketball for her high school. She wants to improve to play at the college level. She notices that the number of points she scores in a game goes up in response to the number of hours she practices her jump shot each week. She records the following data:

x (hours practicing jump shot) y (points scored in a game)
5 15
7 22
9 28
10 31
11 33
12 36
Table 12.2

Construct a scatter plot and state if what Amelia thinks appears to be true.

A scatter plot shows the direction of a relationship between the variables. A clear direction happens when there is either

  • high values of one variable occurring with high values of the other variable or low values of one variable occurring with low values of the other variable, or
  • high values of one variable occurring with low values of the other variable.

You can determine the strength of the relationship by looking at the scatter plot and seeing how close the points are to a line (see figures above). When you look at a scatter plot, you want to notice the overall pattern and any deviations from the pattern.

In this chapter, we are interested in scatter plots that show a linear pattern. Linear patterns are common. The linear relationship is strong if the points are close to a straight line. If we think the points show a linear relationship, we draw a line on the scatter plot. This line can be calculated through a process called linear regression. A linear regression line models the trend of the data. However, we only calculate a regression line if one of the variables helps explain or predict the other variable. If x is the independent variable and y is the dependent variable, then we can use a regression line to predict y for a given value of x.