Chapter 4 – Measures of Spread

Measures of spread gauge how spread out the scores are in a distribution. In other words, they measure how different the scores are from each other. If the individuals in a group all scored very close to each other, then the spread is relatively small. If the individuals in a group scored very differently from one another, then the spread is relatively large.

So let’s suppose that you earned a 65 out of 100 on the test, and the average for the class was 65. Your feelings about your score have a lot to do with how you feel about being average in the class. If you wanted to be one of the higher performers in the class, you might be a bit bummed. But what if everyone in the class earned a 65 out of 100 on the test? In other words, there was absolutely no spread in the distribution because everyone scored exactly the same. It might cause you to feel a little less upset. In this way, it is important to have a measure of the spread of the scores (in addition to a measure of center) in order to really understand how the group performed.

Like measures of center, there are a couple of different measures of spread that we can use: range, variance, and standard deviation.