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

Module Content Objectives Background Comparing Means Attrition Versus Loss to Followup New Information: Change Scores and Participation How To Construct Scatter Plots How To Interpret Scatter Plots Linear Relationship Strength of the Relationship: Pearson's Correlation Coefficient What Do All the Numbers and Plots Mean? Adding Points to a Scatter Plot Lessons Learned: What Jack Should Have Asked His Evaluator What Did Jack Learn About His Program From These Analyses? How Can Jack Use This Statistical Information to Improve His Program? How Can Jack Use This Statistical Information To Better Promote His Program? Module 4 Quiz

New Information: Change Scores and Participation

I told Jack that he should have his evaluator look into whether there was a relationship between the amount of services received and the amount of change in alcohol scores. Before we look at the new information, let's talk a little about change scores and the participation scale the evaluator used.

Change Scores

Change scores may seem a bit confusing, but the concept is fairly basic. We look at the difference between the posttest scores and the pretest scores for the participants' alcohol use. To calculate a change score, we subtract the pretest score from the posttest score for each participant.

Positive values denote an increase in alcohol use scores from pretest to posttest. Negative values denote a decrease in alcohol use. A zero value indicates that alcohol use remained unchanged.

Here is an example of a change score calculation:

1. Posttest score - Pretest score = change
2. 0 days uses - 4 days uses = -4 (change score)

This would be an ideal outcome. The participant's change score is negative, meaning this kid drank much less after the program than he did before.

Participation Scale

The evaluator coded attendance per participant based on the following 5-point scale:

1=1-36 days
2=37-72 days
3=73-108 days
4=109-144 days
5=145-180 days

For example, if a participant attended 108 days (3 times a week), he or she would be coded as a 3. If a participant attended all the sessions, he or she would be coded as a 5.

The next step is to look at change scores and attendance. For example, one participant had a change score of -2 and an attendance score of 3. Another had a change score of 3 and an attendance score of 0. Looking at change scores and attendance will help us answer the question:

Is there a relationship between services received during the Cool After School Program and the change in alcohol use?

Change Scores, Alcohol Use

Kid #	Pretest	Posttest	Change in Alcohol Use (Post - Pre)	Attendance Scale (0-5)

Let's start by plotting the data.