Data Matters with SPSS®

Activity 8.3

A scatter plot is like a statistic. When a researcher gathers data and summarizes it in a scatter plot, there are similarities to what happens when that researcher summarizes the data with proportions and averages.

To see this, in this project, you will take samples of the representative U.S. sample and get scatter plots from those samples. As you replace the samples, you will somewhat see how random sampling affects scatter plots.

Open RepUSSample.sav.

Getting a Scatter Plot of 20 Random Observations

In the data editor, click on Data, then select Select Cases, Sample, Exactly. Fill in the boxes so the sentence is “Exactly 20 cases from the first 50000 cases. Click on Continue, OK.

Click on Graphs, then select Scatter, Define. Select the variable you want as the x-axis and click on the black triangle next to the X axis box. Do the same to assign the y-axis variable. Click OK.

Repeat the steps, starting back at clicking on Data in the data editor. As you repeat the steps, you will see that it goes fairly quickly because several boxes are already filled in.

See what happens to the scatter plot. Try other pairs of variables.

Try increasing the sample size to 200. In the sample window, edit the sentence to “Exactly 200 cases from the first 50000 cases.”

How does the increased sample size influence how random sampling affects the scatter plot?

What is the relationship between the image seen in a scatter plot of a sample’s data and the image seen in a scatter plot of the entire population’s data?