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 68-95-99.7 Percent Rule

Remember how standard deviations can help you understand the way your data are distributed without looking at each piece of data separately? For example, you could understand the distribution of marijuana scores without looking at all 50 scores. If your data are normally distributed, the 68-95-99.7 rule is a good way to remember the pattern the data follow. This is because all normal distribution curves have the following properties:

• 68 percent of the observations fall within 1 standard deviation of the mean.
• 95 percent of the observations fall within 2 standard deviations of the mean.
• 99.7 percent of the observations fall within 3 standard deviations of the mean.

Thus, for a normal distribution, almost all values lie within 3 standard deviations of the mean. This rule is illustrated below. Click the buttons to view each category.