Analyzing Within-Subjects Designs with Three Levels Using SPSS

Study Notes

Analyzing Within-Subjects Designs with Three Levels Using SPSS

Within-subjects experiments involve measuring participants multiple times under each condition of interest. One common situation is when we have three conditions (levels) and want to analyze the data using SPSS. In such cases, we can use repeated measures ANOVA (Analysis of Variance), which is appropriate since our observations are dependent. Here's how you can perform this analysis within SPSS:

Step 1: Prepare Your Data

First, ensure your data is organized correctly. If necessary, rearrange it so that all measurements occur before their respective repeated measure(s).

Data organization

In this example, each row represents one participant, and columns represent different time points where the measurement was taken. All measures from the same time point fall into the same column. Each person has had all three treatments.

Step 2: Set Up the Repeated Measures Analysis

After preparing your data, go to the menu bar and select "Analyze" -> "General Linear Model" -> "Repeated Measures". Then click on "Define Factors...".

SPSS General Linear Model Menu

Here, enter the names of the factors and the number of levels for each factor. Remember to put the name of the variable in the box labeled "Factor Name", and the numbers of levels in the boxes labeled "Number of Levels". Also, make sure the correct factor is highlighted.

In our case, we only have one factor called "Condition" with three levels. So, we will input "Condition" and "3" accordingly.

Setting up Factor in SPSS

Step 3: Add Variables to the Model

Now, you need to specify the variables you want included in the model. Highlight the named factor (in our case, "Condition"), and then click on "Add". This will bring up a new window where you can specify the variables to include in the model.

Adding Variable to the Model in SPSS

For each level, choose the variables corresponding to that level. Since there are three levels (A, B, C), we will choose the variables associated with these levels.

Choosing Variables in SPSS

Step 4: Run the Analysis

Once you have added the variables to the model, click "OK" to run the analysis. SPSS will now calculate the ANOVA table and display the results.

SPSS Output for Within Subject Design

In our output, you can find the significant main effect of Condition, indicating there is a statistically significant difference among the three conditions. Additionally, the interaction terms (Condition x Time) are also significant, meaning the effects of condition vary across time.

Description

Learn how to analyze within-subjects experiments with three levels using SPSS software. Follow steps to prepare your data, set up the repeated measures analysis, add variables, and interpret the results generated by SPSS. Understand how to perform repeated measures ANOVA for dependent observations.