Introduction & Practice Data File

There's 3 ways to create boxplots in SPSS:

• Graphs Legacy Dialogs Boxplot
• Analyze Descriptive Statistics Explore
• Graphs Chart Builder

The first approach is the simplest but it also has fewer options than the others. This tutorial walks you through all 3 approaches while creating different types of boxplots.

• Boxplot for 1 Variable - 1 Group of Cases
• Boxplot for Multiple Variables - 1 Group of Cases
• Boxplot for 1 Variable - Multiple Groups of Cases
• Tip 1 - Remove Outliers for Single Group
• Tip 2 - Show Outlier Values in Boxplot
• Tip 3 - Adding Titles to Boxplots

Example Data

All examples in this tutorial use driving-test.sav, partly shown below.

Our data file contains a sample of N = 238 people who were examined in a driving simulator. Participants were presented with 5 dangerous situations to which they had to respond as fast as possible. The data hold their reaction times and some other variables.

Boxplot for 1 Variable - 1 Group of Cases

We'll first run a boxplot for the reaction times on trial 1 for all cases. One option is Graphs Legacy Dialogs Boxplot which opens the dialogs shown below.

Completing these steps results in the syntax below.

Result

Our boxplot shows some potential outliers as well as extreme values. Interpreting these -and all other boxplot elements- is discussed in Boxplots - Beginners Tutorial. Also note that our boxplot doesn't have a title yet. Options for adding it are discussed in Tip 3 - Adding Titles to Boxplots.

Boxplot for Multiple Variables - 1 Group of Cases

We'll now create a single boxplot for our 5 reaction time variables for all participants. We navigate to Analyze Descriptive Statistics Explore and fill out the dialogs as shown below.

“Dependents together” means that all dependent variables are shown together in each boxplot. If you enter a factor -say, sex- you'll get a separate boxplot for each factor level -female and male respondents. “Factor levels together” creates a separate boxplot for each dependent variable, showing all factor levels together in each boxplot.

“Exclude cases pairwise” means that the results for each variable are based on all cases that don't have a missing value for that variable. “Exclude cases listwise” uses only cases without any missing values on all variables.

A minor note here is that many SPSS users select “Normality plots and tests” in this dialog for running a

• Kolmogorov-Smirnov normality test and
• Shapiro-Wilk normality test.

Anyway. Completing these steps results in the syntax below. Let's run it.

Result

Now, before inspecting our boxplot, take a close look at the Case Processing Summary table first.

The first columns tells how many cases were used for each variable. Note that trial 5 has N = 205 or 86.1% missing values. Remember that “Exclude cases listwise” was the default in the Explore dialog. If we hadn't changed that, then none of our variables would have used more than N = 33 cases. The actual boxplot, however, wouldn't show anything wrong. This really is a major pitfall. Please avoid it.

Anyway, the figure below shows our actual boxplot.

Note that we already saw the first boxplot bar in our previous example. Second, trials 2 and 4 seem strongly positively skewed. Both variables look odd. We'd better inspect their histograms to see what's really going on.

Boxplot for 1 Variable - Multiple Groups of Cases

We'll now run a boxplot for trial 3 for age groups separately. We first navigate to Graphs Chart Builder and fill out the dialogs as shown below.

Select “Point ID Label” in this tab and then drag & drop r03 into the ID box on the canvas. Doing so will show actual outlier values in the final boxplot.

Completing these steps results in the syntax below.

Result

This boxplot shows increasing medians and standard deviations with increasing ages. Note that our boxplot also shows outlier values. In this example, these are reaction times of 1,441 and 1,455 milliseconds but for the youngest age group only.

Tip 1 - Remove Outliers for Single Group

If you'd like to remove outliers based on boxplot results, you'd normally set them as user missing values. For example, MISSING VALUES r03 (1441 THRU HI). sets values of 1441 and higher as missing for r03. In our example, however, this won't work: the aforementioned values are potential outliers only for the youngest age group. For the other age groups, they're within a normal range.

A solution is converting these values into different values for the youngest age group only. One option is combining DO IF with RECODE. The syntax below, however, shows a shorter option based on IF.

Tip 2 - Show Outlier Values in Boxplot

You can show data values for potential outliers and extreme values in boxplots. This only works if each boxplot involves a single dependent variable. Simply use this dependent variable as the ID variable too.
The only dialog that supports this is the Chart Builder. If you prefer the other dialogs, modifying the /ID subcommand in the syntax also does the trick.

Tip 3 - Adding Titles to Boxplots

There's 3 options for showing titles in SPSS boxplots:

• create your boxplot via the Chart Builder as in example 3;
• use a chart template that has a fixed title and/or subtitle;
• add a title manually after creating your boxplot.

For this last option, open a Chart Editor window by double-clicking your chart. You can now add a title from the Options menu.

Note that you can adjust your title after adding it.

Final Notes

There's many more variations on boxplots, especially clustered boxplots. However, I think you'll get them done fairly easily after studying this tutorial.

If you've any questions or remarks, please throw me a comment below.

Thanks for reading!