Reading: SPSS Base 9.0: Chapter 12, Descriptives
Homework: none
If you have continuous variables, then you can use DESCRIPTIVES to calculate summary statistics. Although most of the same statistics can be calculated with the FREQUENCIES procedure, the DESCRIPTIVE procedure is more efficient because it doesn't sort the values into a frequencies table.
The mode and the median are computed from sorted values, so they are not available within DESCRIPTIVES.
The general strategy for running any SPSS procedure is as follows:
| Select the Procedure |  |
Select the Variables | |
Select the Options | |
| Run the Procedure | |
Interpret the Output | |
Save or Print the Output |
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The descriptives dialog box is opened by clicking
Analyze
Descriptive Statistics
Descriptives
The dialog box shows the variable names in the data set in the left box. The variables to be analyzed will be moved to the empty box to the right. Options are accessed by the button at the bottom of the dialog window. The buttons to the far right will run the procedure, paste the commands into the syntax window, reset the variables to be analyzed, cancel the procedure, and provide help.
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The variables to be analyzed are selected by highlighting one or more variables and then clicking the right arrow button between the two boxes. The selected variables move to the Variable(s): box at the right. Note that the selected variables no longer appear in the variable list box at the left.
You can highlight a single variable by moving the cursor to a variable and pressing the left mouse button.
Selecting Multiple Variables
Multiple variables can be selected in one of three ways.
a) Selecting another variable and moving it to the Variable(s): box.
b) You can press and holding the ctrl key while selecting a set of variables and then move the entire set to the Variable(s): box. For example you could select all the knowledge of statistics items for analysis by holding down the ctrl key while selecting the variables for knowledge of anova, chi square, correlation, factor analysis, frequency distribution, multiple regression, repeated measures anova, and t test and then press the right arrow key to move the entire set to the Variable(s): box.
c) If the variables you wish to select are contiguous to each other you can pressing and holding the shift key while selecting a set of variables. For example you could select all the variables for analysis by clicking the variable at the top of the list, pressing and holding the shift key, and then clicking the variable at the bottom of the list. Or you could hold down the shift key while moving the cursor down the variable list. The variables are ordered alphabetically so this method may not be the preferred method of selecting multiple variables. For example, one could not select the knowledge of statistics questions using this selection method.
If you wanted to run descriptives on nearly all the variables you could use the shift key to select all the variables and then use the ctrl key to unselect the unwanted variables. For example, suppose that you wanted to run descriptives on all the variables except ID. First, use the shift key to select all the variables and then press and hold the cltr key while you click on the ID variable.
Run the Procedure
Once a variable is selected the OK and Paste buttons become available. Press the OK button to run the descscriptives procedure on the selected variable(s).
Interpret the Output
The results are shown in an Output Navigator Window. The left pane of the output window contains an outline of the information, the right pane contains the data.
The output is divided into three sections: title, notes, and the descriptive statistics for all the variables selected.
Title. The default title is the name of the procedure that was run. In this case the title is Descriptives. You can edit the title by moving the cursor to the title and then double clicking the left mouse button.
Notes. The notes section contains information about when the statistics were run, some information about the data file, how missing values were handled, the syntax commands that were generated, and the resources used. The notes section is normally closed (the icon next to "Notes" is a closed book rather than an open book). You can open (display) the notes section by double clicking on the closed book icon in front of "Notes." Double click on the open book icon to close the Notes section.
Descriptive Statistics. The statistics section includes statistics for all the selected variables. The default statistics include the number of valid cases, the minimum and maximum values, the mean, and the standard deviation.
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The statistics and display options are all included under the Options.. button at the bottom of the Descriptives dialog box.
Measures of central tendency (mean and sum). You should all recognize these central tendency measures. The mode and median are available in the frequencies procedure.
Measures of dispersion (standard deviation, variance, range, minimum, maximum, and standard error of the mean). These measures are also very common. The standard error of the mean is found by dividing the standard deviation by the square root of the number of valid cases.
Measures of distribution (skewness and kurtosis). The terms skewness and kurtosis refer to distribution shapes that deviate from the shape of a normal distribution. See the discussion of skewness and kurtosis in the frequencies procedure notes.
The variables can be displayed in several different orders: according to order that you selected them (the variable list option); alphabetical; or by ascending or descending means.
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Z scores are standard scores. The formula for computing a z score is
X
- M
z =
--------
S.D.
where X is the raw score, M is the mean, and S.D. is the standard deviation.
Because the mean and the standard deviation of z scores are standardized, the mean = 0 and the standard devation = 1, they are useful for comparing scores across different variables, e.g., comparing a mathematics score with a history score.
The descriptives procedure can create z scores for you. Those z scores can then be used in other procedures.
You can create z scores for the selected variables by checking the Save standardized values as variables box in the descriptives dialog window.
The new z scores will be called "z" + "the first seven letters of the source variable name." For example if you selected variables called corr and ttest, the new z scores would be called zcorr and zttest. The new z scores are added to the end of the current file in the Data Editor.
You can verify that the z score have been added to the current data file with the variables command
Utilities
Variables...
The variables command displays the names and positions of all the variables in the active file. Notice that the z score variables are placed at the end of the variable list. You can quickly goto any variable in the list by highlighting the variable name and clicking the Go to button. (The Paste option in the Variables window will paste the variable name into the Syntax Window.)
Try running descriptives on z scores. Are the means = 0 and the standard deviations = 1?
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You can save the syntax commands that SPSS creates by clicking the Paste button on the right side of the Frequencies dialog box. The syntax editor window will open displaying all the commands for the current settings. You can save the commands in the normal way from the syntax editor window. Saving the commands will allow you to easily run the same set of commands again without having to select all the options from the various dialog boxes.
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The output displayed in the Output Navigator window can be printed or saved to a disk.
Print the output. Select the output to be printed by highlighting elements in the outline pane. The highlighted elements with the open book icon will be saved. You can open or close the book on an individual element by double clicking on the book icon at the front of each element.
You can preview how the document will look when printed by selecting
File
Print Preview
Save the output as an *.spo file. If you press the disk icon or
File
Save
then the highlighted elements in the outline pane will be saved. As with the print option, only those highlighted elements with the open book icon will be saved. The default file extension is *.spo (SPss Output). As far as I can tell the *.spo file can only be open in the SPSS Output Navigator window. Use the
File
Open
sequence in SPSS to open the *.spo file. You cannot open an *.spo file in your favorite word processor.
The Save As option gives you the option of saving the file as a Navigator file (*.spo) or as "all files (*.*). However, saving the output navigator file with the *.doc extension still will not allow the file to be opened successfully with MS Word or with MS WordPad.
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You can save an SPSS 9.0 Viewer object directly into a word processor such as MS Word 95/97 by (a) right clicking the object, (b) press the copy option, (c) open a Word 95/97 file and then paste the object into the open file. The object is created as a normal table in Word 97. You can edit the elements of the table within Word 95/97.
It is also possible to use the copy objects command rather than the copy command to copy and paste an object from the SPSS Viewer to a Word 95/97 file. The object is created as an object in Word 95/97. Although it is possible to edit the contents of an object in WS Word 95/97, I have found the process to be tedious and unreliable. It is easier to do all your editing in the output navigator prior to copying the objects to MS Word.
In summary, I recommend using copy and paste to move objects from the SPSS Viewer to a Word 95/97 file.
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ŠLee A. Becker, 1997,1998 -revised 09/13/99