The sixth edition of the Publication Manual of the American Psychological Association offers guidance for writers and publishers in the social and behavioral sciences. In general, figures are used for numbers 10 and above, and numbers below 10 are spelled out. However, there are some exceptions.
APA Rules for Numbers 10 and Above
The sixth edition of the APA style guide states that figures should be used to represent numbers 10 and above (e.g., 15 of the participants). Figures are also used to group numbers above and below 10 together for comparison (e.g., 4 of 26 groups); to depict time, dates and age (e.g., 5 years ago, 6 hr 30 min); and to denote a location in a book or table (e.g., page 19, Table 4).
APA Rules for Numbers Below 10
When a number below 10 does not represent a precise measurement, APA style instructs writers to use words to denote these numbers (e.g., six pages). Additionally, words should always be used to represent numbers when establishing a new sentence, title or heading (e.g., Ninety percent of the people responded to the survey).