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QuestionI want to analyze data from a government survey. I know that I need to use the SPSS Complex Samples module to appropriately handle the survey design. I was able to obtain the data file in SPSS format from a U.S. government agency Web site, but I'm not sure how to specify my design in SPSS Complex Samples. How do I do this?
AnswerYou will need to check with the agency publishing the data to make sure you know the properties of the sampling design. Most software, however, does not fully account for multistage sampling designs, therefore many U. S. government survey publishers make the data available in a form that is most appropriately handled as a single-stage design, using with-replacement (WR) estimation.
One widely used survey, for example, is the Youth Risk Behavior Survey (YRBS), part of the Youth Risk Behavior Surveillance System (YRBSS) of the Centers for Disease Control and Prevention (CDC). Data are produced via a three-stage cluster sampling design, with stratification at the first stage. The data made available for public use, however, are designed and weighted to be used as if the design were a single-stage cluster sampling design with stratification and sampling WR.
According to the CDC, this approach leads to underestimation of variances by three to 10 percent when compared with use of all design characteristics. Were data made available with more details about subsequent stages, SPSS would be able to provide more accurate results. At the current time, however, for this and many other surveys, the single-stage WR specification is the best available option.
It’s simple to create the analysis plan (.csaplan) file necessary to analyze the YRBS data in the SPSS Complex Samples module using the CDC recommended approach. The yearly YRBS SPSS (.sav) data files made available by the CDC (at http://www.cdc.gov/HealthyYouth/yrbs/data/index.htm) contain three variables—weight, stratum, and psu—necessary to completely produce the analysis plan. These variables are near the end of the file. To create an analysis plan file, simply do the following (see Figure 1 below):
Your analysis plan file is now complete and can be used whenever you want to perform your analyses. When you select one of the analysis procedures in the SPSS Complex Samples module from the SPSS menus, you will see a dialog box that asks you to specify an analysis plan file. Simply specify the file you've just created and click Continue. The main dialog box for the analysis procedure will appear, and you can specify your analysis without worrying further about the sampling design.
Figure 1
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