In general, use words to express numbers zero through nine, and use numerals to express numbers 10 and above.
there were five nurses on duty
the study had three conditions
students were in the third, sixth, eighth, 10th, and 12th grades
Numbers expressed in words are covered in Section 6.33 of the APA Publication Manual, Seventh Edition
However, there are exceptions to this general guideline for number usage. There are cases in which you should always use words to express numbers, even numbers 10 and above, and likewise, there are cases in which you should always use numerals to express numbers, even numbers zero through nine.
Cases in which to always use words for numbers
Always use words to express numbers in the following cases, even numbers 10 and above:
Case |
Example |
---|---|
Numbers that begin a sentence, title, or heading (when possible, reword the sentence to avoid beginning with a number) |
Fifty percent of the students received the intervention, and the other 50% were part of a control condition. Twenty people enrolled in the class, but 15 dropped out. |
Common fractions |
one fifth of the class two-thirds majority |
Certain universally accepted phrases |
Twelve Apostles Five Pillars of Islam |