The Grammar of Mathematics: Percentage or %?

by Timothy McAdoo

As Chelsea so succinctly noted in her recent post about how statistical terms are introduced and used in APA Style manuscripts, “in the social sciences, the worlds of grammar and mathematics intersect.” Thus, when you first start to write about statistical results, you may encounter style questions that you’ve not considered before. In today’s post, I answer one such question:

Question: How do you decide whether to use the percentage symbol (%) or the word percentage?

Answer: Use the symbol only when it is preceded by a numeral; otherwise, spell out the word percentage.

For example,

What percentage of wood would a woodchuck chuck if a woodchuck could chuck wood? In Experiment 1, we used a computer simulation to address this timeless question. The woodchucks (who would chuck) chucked 86.4% of the wood available during the test. This was a larger percentage than we hypothesized. Two woodchucks (33.3% of the virtual subjects) would not chuck wood (see Table 1).

You’ll find these guidelines on page 118 of the Publication Manual. On the same page, the Manual also notes just one exception: "In table headings and figure legends, use the symbol % to conserve space."