Here is a kind of mistake so common it should always prompt a copyeditor to stop and check:

*The number for 2013 represents a .02 percent increase over the number for 2012.*

We don’t have all the information, so it could be right; but it’s probably wrong. Some simple consideration of scale and possibly some mental math will help us know for sure.

It’s common for copyeditors to shun math. We don’t trust our writers with words, yet we assume they know what they’re talking about where numbers and percentages are concerned. We skip over the mathy parts in our quest for misplaced commas. We shouldn’t. Math and numbers present plenty of opportunities for error.

Math calculations involving percentages can be expressed as decimals, or per 1. It’s easy to forget to move the decimal when the word “percent” is added. When a copyeditor sees a number in the hundredths (with two decimal places after the period) and “percent,” that’s a cue to pay close attention.

If we’re talking about budget or profit increases that are essentially flat, the .02 percent in our example is possible. If we’re comparing two vastly different scales, it’s possible: a 5-mile walk is .02 percent of a walk around the world.

If we’re talking about a change in the number of employees of a local school district, .02 percent is less likely. Say there are 500 employees. Two percent means 10 people. Two-tenths of a percent, then, is one person and .02 percent is one-tenth of a person.

I recently came across this type of error while editing a statistical analysis of a relatively small survey. Only a very large survey would generate a percentage with two decimal places, so it was an easy catch.