Stats: Watch Your Math

Be careful with numbers. Make sure you say what you mean, and you mean what you say, and that you understand what you say.

For example, let's look at this passage:

The U.S. Census Bureau ... (found) 61.8 percent have computers, an increase of almost 54 percent since 1984.

This is a fatal.

How is that? you may say. It went from 8.2 percent in 1984 to 61.8 percent now. The difference is 53.6 percent!

That's because the difference in percentage points is 53.6 percent. But the difference in percentage growth is actually 653.6 percent!

Here's what I mean:

In 1984, 8.2 percent of 100 percent households had computers. If 100 percent is 113.1 million households, that means 8.2 percent is around 9.2 million households.

Today, 61.8 percent of that 113.1 million households have computers. 61.8 percent of 113.1 million is around 69.9 million.

So the percentage increase isn't 8.2 to 53.6; it's roughly 9.2 million to somewhere around 69.9 million. And that's an increase of over 650 percent. If the 9.2 million only went up just over 53 percent, we'd be talking about a total of around 14 million or so.

What I think you meant to say was that the percentage of households with computers has risen 53.67 percentage points. Which it did. But that's not what you said.

If you're not sure, check with your sources to make sure your math is correct and in proper context.

Nw, I understand math is hard. That's why many of us went into writing; to get away from math, right?

Still, we have to know how to accurately calculate percentage change, and these day it's never been easier with the Internet. Just do a Google search for "percentage change calculator" and you'll find dozens. That's how I did my math.

By the way, this isn't an isolated mistake. In my class last fall someone made the exact same mistake as you did here.

So don't fret. Do work on remembering the lesson, and not repeating the mistake.