RANDOM SAMPLING IN SPSS

This first step - changing the Random Number Seed - is not necessary in order to draw a random sample in SPSS.  The only reason I have you set the Random Number Seed to the exact number as everybody else in the class is to insure that everybody will have the same cases and thus the same statistics in the sample.  Here's how to do it:

Select Transform... Random Number Seed.

Then click the Set seed circle to choose this option:

Then enter the particular number I have told you to enter, for example, 30.  Then click OK.

Now, you are ready to draw a random sample of cases.  Select Data... Select Cases:

In the Select Cases dialog box, choose Random Sample of Cases, then click the Sample rectangle below:

There are two ways to pick a random sample of cases.  You can either select a certain percentage of cases or a certain number of cases.  Here's how to select a certain percentage of cases - approximately 25%:

Click Continue in Select Cases: Random Sample and OK in the Select Cases dialog box (below).  Note that next to the Sample... rectangle, there's a confirmation that you are sampling approximately 25% of the cases.

If you switch to data view after selecting the sample, you will notice that there's a slash beside some cases in the gray column to the left.  These are cases that have NOT been selected into the sample.  The unslashed cases are in your sample of approximately 25% of the cases.

To select another different random sample of approximately 25% of cases, you would return to Data... Select Cases...  At this point, all you have to do is select OK, and you will select a second sample of approximately 25% of cases:

You can continue in this manner to select as many different random samples of approximately the same percentage of cases as needed.

The second method of selecting a random sample of cases is to select a certain number of cases.  Go back to the Data... Select Cases dialog box, and again click the Sample... rectangle under Random sample of cases.  Choose Exactly (blank) cases from the first (blank) cases:

In the first blank box, enter the number of cases I have told you to select for your sample - e.g., 200.  In the second blank box, enter a number that is equal to the total number of cases in the data file (the "population" in my examples):

After clicking Continue in the Select Cases: Random Sample box and OK in the Select Cases box, you sample is ready for analysis!  If you need to select another sample of 200 cases, just return to Data... Select Cases... and click OK.

In order to return to analyzing the full sample, choose Data... Select Cases... All cases and click OK:

The slash marks will now disappear in the gray column to the left, and you are ready to conduct an analysis on all cases in the data file.