Communicating Data with Tableau

Chapter 4. Ratios and Rates

“We are the safest large city in America, but any crime rate is too high.”

—Michael Bloomberg, former mayor of New York City

Until now, we’ve considered absolute amounts of countable and measurable nouns, like population, tons of garbage collected, and number of rat sightings by borough. Pleasant, I know.

But not all boroughs are created equal. As we saw in Chapter 1, they have different surface areas and different numbers of people living in them. The fact that they produce different amounts of garbage isn’t surprising, and probably doesn’t make for a very interesting message to communicate to an audience.

These absolute comparisons, while helpful for showing the big picture, aren’t generally considered “apples-to-apples” comparisons. There are apples, and, well, there are Big Apples.

A helpful way to “normalize” comparisons is using ratios, rates, proportions, and percentages. What’s the difference between these four?

• A ratio is a comparison of two terms expressed as a quotient. For example, Manhattan produced 0.264 tons of recycle for every ton of refuse. Ratios can be expressed as “x to y,” “x:y,” “x/y,” or as a decimal.

• A rate is a ratio in which the two terms have different units. For example, the population density of Brooklyn is 36,136 residents per square mile. Rates are often predictive because time can be used as the denominator (crime rates, population growth rates, etc.).

• A proportion is a ratio in which the numerator is a partial amount and the ...