Page 207 - Statistics II for Dummies

Chapter 11

Finding Your Way through

Two-Way ANOVA

In This Chapter
▶ Building and carrying out ANOVA with two factors
▶ Getting familiar with (and looking for) interaction effects and main effects
▶ Putting the terms to the test
▶ Demystifying the two-way ANOVA table

nalysis of variance (ANOVA) is often used in experiments to see whether
Adifferent levels of an explanatory variable (x) get different results on
some quantitative variable y. The x variable in this case is called a factor, and
it has certain levels to it, depending on how the experiment is set up.

For example, suppose you want to compare the average change in blood
pressure on certain dosages of a drug. The factor is drug dosage. Suppose
it has three levels: 10mg per day, 20mg per day, or 30mg per day. Suppose
someone else studies the response to that same drug and examines whether
the times taken per day (one time or two times) has any effect on blood pres-
sure. In this case, the factor is number of times per day, and it has two levels:
once and twice.

Suppose you want to study the effects of dosage and number of times taken
together because you believe both may have an effect on the response. So
what you have is called a two-way ANOVA, using two factors together to
compare the average response. It’s an extension of one-way ANOVA (refer to
Chapter 9) with a twist, because the two factors you use may operate on the
response differently together than they would separately.

17_466469-ch11.indd 191 7/24/09 9:44:16 AM