# How SPSS Works

The developers of IBM SPSS Statistics have made every effort to make the software easy to use. SPSS prevents you from making mistakes or even forgetting something. That’s not to say it’s impossible to do something wrong in SPSS, but the SPSS software works hard to keep you from running into the ditch. To foul things up, you almost have to work at figuring out a way of doing something wrong.

You always begin by defining a set of *variables*; then you enter data for the variables to create a number of *cases*. For example, if you’re doing an analysis of automobiles, each car in your study would be a case. The variables that define the cases could be things such as the year of manufacture, horsepower, and cubic inches of displacement. Each car in the study is defined as a single case, and each case is defined as a set of values assigned to the collection of variables. Every case has a value for each variable. (Well, you *can* have a missing value.)

Each variable is a specific type. Types describe how the data is *stored* — for example, as letters (strings), as numbers, as dates, or as currency. Each variable is defined as containing a certain kind of number, so you also have to define the variable’s level of measurement.

For example, a *scale* variable is a numeric measurement, such as weight or miles per gallon. A *categorical* variable contains values that define a category; for example, a variable named gender could be a categorical variable defined to contain only values 1 for female and 2 for male. Things that make sense for one type of variable don’t necessarily make sense for another. For example, it makes sense to calculate the average miles per gallon, but not the average gender.

After your data is entered into SPSS — your cases are all defined by values stored in the variables — you can easily run an analysis. You’ve already finished the hard part. Running an analysis on the data is simple compared to entering the data. To run an analysis, you select the analysis you want to run from the menu, select the appropriate variables, and click OK. SPSS reads through all your cases, performs the analysis, and presents you with the output as tables or graphs. Of course, you have to know which analysis to choose.

You can instruct SPSS to draw graphs and charts directly from your data the same way you instruct it to do an analysis. You select the desired graph from the menu, assign variables to it, and click OK.

When you’re preparing SPSS to run an analysis or draw a graph, the OK button is unavailable until you’ve made all the choices necessary to produce output. Not only does SPSS require that you select a sufficient number of variables to produce output, but it also requires you to choose the right kinds of variables. If a categorical variable is required for a certain slot, SPSS won’t allow you to choose any other kind of variable. Whether the output makes sense is up to you and your data, but SPSS makes sure that the choices you make can be used to produce some kind of result.

All output from SPSS goes to the same place — a dialog box named SPSS Statistics Viewer. This dialog box displays the results of whatever you’ve done. After you’ve produced output, if you perform some action that produces more output, the new output is displayed in the same dialog box. And almost anything you do produces output.