CONTENTS

Table of Contents
Introduction

1. Descriptive Statistics in Evaluation
2. Subgroup Analysis
3. Variable - Are They Related?
4. Correlation
5. The t-Test of Difference Between Means

Supplements
Resources
Glossary

Wading Through the Data Swamp:
Program Evaluation 201

The t-Test of Differences Between Means:
Independent Samples t-Test for Posttest Data

Step 9: Calculate the combined variance.

At this point we need to know the variance because it is part of the t-test formula.

The formula is:

Since we have two groups, this step involves adding the two sums of squares calculated in step 7 and dividing by the combined degrees of freedom calculated in step 8. The SS for the participant group plus the SS for the comparison group is 288.88 + 544.5 = 833.38. If we divide the combined sums of squares, 833.38, by the combined degrees of freedom, 98, it will give a combined variance of 8.5.