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SPSS Exercise Descriptive Statistics

This example is based on a study by Foa, Rothbaum, Riggs, and Murdock (1991) in the Journal of Counseling and Clinical Psychology. The subjects were 45 rape victims who were randomly assigned to one of four groups.

The four groups were:

1) Stress Inoculation Therapy (SIT) in which subjects were taught a variety of coping skills (similar to cognitive therapy);

2) Prolonged Exposure (PE) in which subjects went over the rape in their mind repeatedly for seven sessions (the idea being the more we are exposed to something the less we have anxiety about it);

3) Supportive Counseling (SC) which was a control group where people were provided with attention and positive regard, but no specific therapy

4) Waiting List (WL) control – no therapy, just measurement.

In the actual study, there were many outcome variables. For our purposes we will only look PTSD Severity, which was the total number of symptoms endorsed by the subject (the sum of the subject’s ratings on about 15 variables related to PTSD—e.g., flashbacks, nightmares, memory difficulties, etc.) Higher scores represent more PTSD symptoms.

People were measured on 3 occasions: Pre-treatment, Post-treatment, and one month following treatment.

To Summarize:

Independent Variables

oBetween Subjects conditions–therapy type (4 conditions)

oWithin Subjects conditions—repeated measures (3 times)

Dependent Variable = PTSD Severity

To begin, open the data file.  (RSM701Week7.sav, click on SAVE AS, once downloaded, open the file with SPSS)

Once the data is open in SPSS, notice the tabs at the bottom, there is a variable view and a data view.

The data view allows you to see all the data (this is where you would enter new data).

The variable view allows you to see the variables (this is where you would create new variables).

PART I: DESCRIPTIVE STATISTICS

We need to explore our data to get a beginning picture of what is happening.

First try ‘Descriptives’

In the menu bar, click on ANALYZE/DESCRIPTIVE STATISTICS/DESCRIPTIVES

A dialog box will open

Highlight the variables, ‘pre’, ‘post’, and ‘follow-up’

Click on the arrow in the middle that moves them to the box on the right

Click on OPTIONS (click on any options that you might want to add)

Click on RUN

Your output file will now open

Q1) The descriptive statistics analyzed in Week 4 combined all four groups.  We want to know how the four groups did in therapy. To accomplish this task, we need to use the EXPLORE command.

Click on ANALYZE/DESCRIPTIVE STATISTICS/EXPLORE

A dialog box will open.

You will need to put your variables of interest into the DEPENDENT LIST box (similar to how you did it in DESCRIPTIVES previously).

You will also need to put the variable ‘group’ into the FACTOR LIST box (A factor is the same thing as an independent variable; Thus, this allows us to break the statistics up by group membership)

Click on RUN

Your output file will now open

The output is not the easiest to read.  The ‘Pre’ descriptive statistics are listed first with each group getting a subsection.  Then the ‘Post’ descriptive statistics are listed with each group getting a subsection. Finally, the “Follow up’ descriptive statistics are listed with each group getting a subsection.

(Optional: If you are interested in making it easier to read, you can right click on the table and go down to SPSS Pivot Tray Object/EDIT, then open PIVOT TRAYS.  This feature allows you to move the rows and columns around, which is very helpful when submitting a table for publication).

Q2) Fill in this table with the means of each group at each time point (round numbers to two decimal points)

Table 1 Means

Group Pre Post Follow-up

Stress Inoculation

Prolonged Exposure

Supportive Counseling

Wait List

Q3) Show me the differences between the groups on the pre- and post-test occasions (i.e., Generate a figure for pre and a figure for post; the Boxplots are nice).

Q4) What can you say about change over treatment? Keep in mind that these are DESCRIPTIVE statistics that we are now examining by group (so your answer should be different than in Week 4).

PART II: INFERENTIAL STATISTICS

One-Way ANOVA

Our first ANOVA will be comparing the 3 exercise groups on the last time point.  We want to see whether the exercise groups have a different amount of PTSD at followup (one month after treatment).  Participants reported different amounts of exercise (low, medium, and high), so we are interested in whether there was a difference for the three exercise groups.

Q5) Write the H0: for the ANOVA comparing 3 groups at follow up

Click on ANALYZE/COMPARE MEANS/ONE-WAY ANOVA

In the dialog box:

o our dependent variable is ‘follow up’

o our factor is ‘exercise’

We can also get a graph of the means of our groups, if we click on OPTIONS and then MEANS PLOT in the next dialog box (note: it is interesting to see how SPSS will automatically generate the y-axis range according to the data, this feature can make a nonsignificant result look significant and a significant result look nonsignificant depending on your data).

We can also compute post-hoc differences to determine which of the groups, if any, differ significantly. Click on Post-hoc and check the box next to Tukey.

Click on OK

Q6) What can you say about the groups one month after treatment (at followup)? Make sure you use the APA notation when describing the statistics (e.g., A significant effect was found for… (F (1, 23) = 2.97, p < .05); ASK in the discussion if you are unsure how to do this.).

Q7) Is there a need to do Post Hoc tests (i.e., do we have an overall difference between groups, where we will need to find which individual groups are different? Was there a significant difference according to results of the ANOVA)? If so, report which groups differ significantly from each other, along with the significance level (p value).

Two-way ANOVA

The main advantage of a factorial ANOVA is that we get to see the two IVs interacting. Thus, we want to run an analysis where we get the interaction term.

Let’s say we are looking at the follow up PTSD scores, and want to determine whether the reduction in PTSD severity is due to the amount of exercise they reported or therapy condition to which they were assigned.

Run an ANOVA on Therapy Group and Exercise

ANALYZE>GENERAL LINEAR MODEL>UNIVARIATE

Place follow up in the DEPENDENT VARIABLE box

Place therapy group AND exercise in the FIXED FACTORS box

Plots are very important when looking at interactions.  Whenever we see plots where the lines are not parallel or they cross, we can be pretty sure we have an interaction.  We can plot this data in two different ways (both plots will give us the same information, but in different formats). For the first plot, click on PLOT and put therapygroup in HORIZONTAL AXIS and exercise in SEPARATE LINES, then click ADD and CONTINUE)

For the second plot, click on PLOT and put exercise in HORIZONTAL AXIS and group in SEPARATE LINES, then click ADD and CONTINUE)

Click OK

There is a lot of output presented.  We can look at the main effect of group and exercise.  However, in factorial ANOVA, we MUST interpret the interaction first.  If there is a significant interaction, then there is no reason to look at the main effects.  That is, the interaction is the only thing that is interesting (if it is significant).

The interaction is listed as therapy group*exercise in the “Test of Between-Subject Effects” table.

Q8) Is there a significant interaction effect of group and exercise on PTSD scores at follow up? Make sure you use the APA notation when describing the statistics (e.g., A significant interaction effect was found for… (F (1, 23) = 2.97, p < .05).).

Q9) Based on the first plot where you have separate lines for exercise, what do you see is happening for the different levels of exercise? Be sure to include the figure and describe what you see.

Q10) Based on the second plot where you have separate lines for group, what do you see is happening for the different levels of therapy group? Be sure to include the figure and describe what you see.

Q11) Is there a significant main effect of therapy group on PTSD scores at follow up? Make sure you use the APA notation when describing the statistics (e.g., A significant interaction effect was found for… (F (1, 23) = 2.97, p < .05).).

Q12) Is there a significant main effect of exercise on PTSD scores at follow up? Make sure you use the APA notation when describing the statistics (e.g., A significant interaction effect was found for… (F (1, 23) = 2.97, p < .05).). Is this similar to or different from the results of the one-way ANOVA? How so?

Q13) Write up the results as if you were writing up a results section like you see in a journal article. Describe what you have found so far in your study. Here are some pointers on what to include:

Please include a brief section on descriptive statistics (i.e., means and standard deviations of PTSD scores at follow up for the different levels of exercise and therapy groups; see Part I on using the EXPLORE command; you will need to put therapy groups and exercise in the FACTOR LIST and followup in the DEPENDENT LIST;

A simple alternative would be to re-run the analysis from above and just click on DESCRIPTIVE STATISTICS in the OPTIONS).

Briefly discuss the results of the pre-test.

Spend most of your analysis reporting the results of the one-way and two-way ANOVAs.

Keep in mind that if you have a significant interaction, you do not need to discuss the main effects. If you do not have a significant interaction, you need to discuss the main effects.

Make sure you use APA formatting for statistical notation, as well as figures and/or table.

ASK in the discussion if you are unsure how to report findings in APA format!