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

Baseline Equivalence of the Cool After School Participants and Nonparticipants

Jack then wanted to make sure that the two groups were comparable. If you want to get a picture of a program's effects, the group that receives services and the group that doesn't should be as equivalent as possible before the intervention. (This is called baseline equivalence.) Otherwise, it's difficult to attribute changes to the program.

Let's say that the comparison group has lower amounts of drug use compared to the experimental group to begin with. They would probably have lower drug use compared to the experimental group after the intervention. Thus, it would look like the program was not effective, when this might not be the case.

Jack's 50 program participants (experimental group) and 50 nonparticipants (comparison group) completed questionnaires addressing each of the measurable questions. Both groups completed the questionnaires before (pretest) and after (posttest) the 9-month prevention program. Jack then had both the participant group and the comparison group fill out pretest surveys. He then had his evaluator determine whether the two groups were equivalent. Fortunately, it was determined that the two groups were equivalent and therefore comparable.

Source: Trochim, W. The Research Methods Knowledge Base, First Edition. Cincinnati: Atomic Dog Publishing, 1999; and Shadish, W.R.; Cook, T.D.; and Campbell, D.T. Experimental and Quasi-Experimental Designs for Generalized Causal Inference. Boston: Houghton-Mifflin, 2002.