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## Chapter 5. Proportions and Percentages

“Baseball is ninety percent mental and the other half is physical.”

Anyone who wants to master the discipline of communicating data needs to learn how to convey proportions and percentages effectively. We see these types of comparisons every day, from the quarterly sales reports to the sports page box scores to the side panels of cereal boxes. Getting them right is important (unless you’re Yogi Berra).

Recall from Chapter 4 that proportions are ratios expressed as values from 0 to 1—where the numerator is a partial amount and the denominator is the total—and percentages are simply ratios expressed as an amount in each hundred. They are most often used to communicate three different types of comparisons:

• Part-to-whole

• Current-to-historical

• Actual-to-target

Let’s consider each one of these types of comparisons one at a time, starting with part-to-whole.

## Part-to-Whole

Just how much of baseball is physical, and how much is mental? If these two aspects of the game are mutually exclusive and collectively exhaustive, they should sum to 100%, right? Yogi’s bogus math in this chapter’s epigraph is funny because it’s different than what we know to expect.

In honor of Yogi, let’s stick with baseball as we consider proportions. In this section, we’ll consider the team batting statistics for the 2012 New York Yankees.

Followers of the sport of baseball have long been fascinated with proportions of outcomes (called stats). As a boy, I would spend all of ...

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