Write the number as a word (i.e., "four" instead of "4") if it is less than 10; if the number is 10 or greater, then write it as a numeral (i.e., "12" instead of "twelve"). The American Psychological Association (APA) endorses this general rule of thumb; however, it also specifies numerous exceptions. Each time you write a number, consider its exact purpose and context---as well as its value---before choosing its form.
Write a number as a word when it begins a sentence. However, try to avoid this situation: rework the sentence structure to begin with a different word. Choose even a passive construction over beginning with a number. For example, "The symptom was displayed by 11 subjects" is better than "Eleven subjects displayed the symptom." However, adding a relevant word or phrase provides a better solution: "Additionally, 11 subjects..." or "The researchers found that 11 subjects..."
Write general and approximate fractions as words. For example, write "half of the participants" rather than "1/2 of the participants." If the fraction is precise or unusual, however, write it in numerals: "11/24," not "eleven twenty-fourths."
Write the number as a numeral if it denotes a specific measurement (i.e., "3.4 inches" or "6 miles") or an exact amount (i.e., "7 participants" or "3 cents"). In particular, express any precise measurement of time in numerals: "5 years," "9 hours," "an 8-year-old boy." Similarly, write the number as a numeral in any mathematical context---for example, when discussing equations or percentages.
Place a zero at the beginning of a decimal numeral less than 1 (i.e., "0.89") unless that type of number could never equal more than 1. (For example, a p value or a proportion never exceeds 1, so omit the 0 before the decimal point.)
Use Roman numerals only when they form an accepted phrase or common usage, such as World War II. Otherwise, use Arabic numerals.
Pluralize a numeral by adding only an "s"; do not add an apostrophe.
Include a comma when the numeral is 1,000 or greater, unless it is a page number, serial number or temperature.