What is Repeated Measures ANOVA and How to Perform It in SPSS?

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What is Repeated Measures ANOVA?

Repeated measures ANOVA is a specialized statistical test used when you take multiple measurements from the same participants across different time points or conditions. It is the extension of the paired t-test to three or more measurement occasions.

When to Use Repeated Measures ANOVA

• Pre-test, post-test, and follow-up designs (same participants measured at multiple time points)
• Crossover trials where participants receive all treatments
• Within-subjects factorial designs
• Learning and development studies tracking change over time

Key Assumptions

• Normality: The dependent variable should be approximately normally distributed at each measurement point
• Sphericity: The variances of the differences between all pairs of measurement conditions should be equal. Tested with Mauchly's test. If violated, use Greenhouse-Geisser or Huynh-Feldt correction.
• No significant outliers at any time point

Step-by-Step Guide in SPSS

1. Go to Analyze → General Linear Model → Repeated Measures
2. In "Within-Subject Factor Name", type a name for your factor (e.g., "Time")
3. Enter the number of levels (time points) in "Number of Levels"
4. Click Add then Define
5. Assign your measurement variables to the within-subjects factors
6. Click Options and select: Descriptive statistics, Estimates of effect size, Observed power
7. For post-hoc tests, click EM Means and select "Compare main effects"
8. Click OK

Interpreting the Output

Key elements to report:

• Mauchly's Test: If p < .05, sphericity is violated; apply correction
• Tests of Within-Subjects Effects: F-statistic, degrees of freedom, p-value
• Partial Eta Squared (η²p): Effect size (small: .01, medium: .06, large: .14)
• Pairwise Comparisons: Which specific time points differ significantly

APA Reporting Example

"A one-way repeated measures ANOVA was conducted to evaluate the effect of time on [outcome measure]. The results showed a statistically significant effect of time, F(2, 58) = 12.45, p < .001, η²p = .30, indicating that scores changed significantly across the three time points."

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