How Much Do You Weigh?

## An Example of Great Reliability

Here's another possible outcome regarding the reliability of your 5 bathroom scales. When you weigh yourself, your 5 scales give different weights. Scale #3 says you weigh only 103 pounds. Scale #4 says you weigh 135 pounds. Which do you believe?

The best answer is to take the average of all five scales. That average is an **index** that one could test for **reliability**. That is, one could compute Cronbach's alpha for the 'bathroom scale index.' Suppose Scale #1 gives the same weight as the (reliable) scale at your doctor's office. While the different scales give different weights, the **average** (your index score) is only a pound less than what you actually weigh.

Again, your friends Jill, Bob, Ellen, and Mike drop by. They try your 5 scales. Each scale provides different weights for everyone. But the average (the **Index **column) is within a few pounds of their actual weight (as shown on Scale #1). The Cronbach's alpha reliability coefficient for these five scales is .92. That indicates that—although the 5 scales give different weights—the average of all five scales gives a generally reliable estimate of each person's true weight.