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

Adding Points to a Scatter Plot

Now that you have learned about relationships between variables, try to plot one the applet below. Just click your mouse and a data point will appear. Notice how the correlation coefficient value (r) changes based on your plot.

Keep in mind that the X (horizontal) axis = Number of Sessions Attended and the Y (vertical) axis = Change in Alcohol Use. So for example, if you were to plot a negative linear relationship (a pattern of scores moving down and to the right) it would mean that as the number of sessions increases, participants' alcohol use decreases. ( This is the pattern that Jack wants to see, with the r value close to -1.)

Notice the r value. The closer the r value is to 1 or -1 the stronger the linear relationship. The closer the r value is to 0, the weaker the relationship.

This applet from Seeing Statistics™ is used under license. Any reproduction or other use is strictly prohibited.

Scatter Plot With Regression Line

Instead of adding data points like you did with the first applet, you will move the regression line (best-fit line) itself, changing the relationship from positive to negative or no relationship. Don't forget to look at the r value.

The labels are the same as they were in the first applet, X (horizontal) axis = Number of Sessions Attended and the Y (vertical) axis = Change in Alcohol Use. Click on the left arrow button to move the best fit line to show a negative linear relationship. Notice how the scores start to move closer to the line.

This applet from Seeing Statistics™ is used under license. Any reproduction or other use is strictly prohibited.