Estimated Marginal Means (EMM) and Post Hoc Tests: Understanding ANCOVA Output in SPSS 25

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In Analysis of Covariance (ANCOVA), Estimated Marginal Means (EMMs) are a crucial aspect for understanding significant effects. These values represent the expected means on the dependent variable if all covariates were at their grand mean, allowing researchers to compare group differences while controlling for confounding variables.

SPSS 25 offers various options and procedures for conducting ANCOVA analyses, including the ability to compute EMMs using its General Linear Model (GLM) procedure. This article will guide you through selecting these options in SPSS 24/25, as well as interpreting the output related to estimated marginal means.

### Step-by-Step Instructions

1. Open your data file and navigate to Analyze > GLM > Mixed Models.

2. In the left-hand dialog box under Fixed Factors:

- Select all covariates that you want to include in the model by clicking on their respective checkboxes next than selecting them again from the Covariance Matrix drop-down menu below.

3. Click on 'Options...' at the bottom of this window, then select 'Estimates' from within it.

4. Checkmark both boxes labeled as 'Include intercept' and 'Continue.' The dialog box should now look like Figure 5 above.

### Understanding ANCOVA Output

Once you have selected these options and run your analysis in SPSS 25, the output will display several tables related to EMMs:

* EMMeans: This table presents estimates of marginal means (i.e., expected values) for each combination of factors at their specified levels. The first column shows the model name followed by all independent variables entered into the equation; rows represent unique combinations thereof based upon how they were defined within 'Fixed Factors.'

5. To access post hoc tests, click on 'Contrasts' under 'Estimation/Assumptions.'

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In conclusion, estimated marginal means are a powerful tool in ANCOVA for understanding group differences while controlling for covariates. By following these step-by-step instructions and interpreting the output correctly using SPSS 25, researchers can gain valuable insights into their data.

Remember to always check your assumptions before conducting any statistical analyses and consult with an expert if you encounter any difficulties or have further questions!

Estimated marginal means spss 25