Chapter 8

# What’s Normal?

IN THIS CHAPTER

Meeting the normal distribution family

Working with standard deviations and normal distributions

Understanding Excel’s normal distribution-related functions

Amain job of statisticians is to estimate population characteristics. The job becomes easier if they can make some assumptions about the populations they study.

One particular assumption works over and over again: A specific attribute, trait, or ability is distributed throughout a population so that most people have an average or near-average amount of the attribute, and progressively fewer people have increasingly extreme amounts of the attribute. In this chapter, I discuss this assumption and what it means for statistics. I also describe Excel functions related to this assumption.

## Hitting the Curve

When you measure something in the physical world, like length or weight, you deal with objects you can see and touch. Statisticians, social scientists, market researchers, and businesspeople, on the other hand, often have to measure something they can’t see or put their hands around. Traits like intelligence, musical ability, or willingness to buy a new product fall into this category.

These kinds of traits are usually distributed throughout the population so that most people are around the average — with progressively fewer people represented toward the extremes. Because this happens so often, it’s become an assumption about how most traits are distributed.

It’s possible to capture ...

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