According to the 6th edition of the "Publication Manual of the American Psychological Association," consistency in style among papers and publications helps prevent confusion for readers. The APA manual explains the specific aspects of style needed to retain consistency, including the formatting of numbers, for papers in sciences and social science fields. In general, Arabic numerals are used for numbers over 20, with some exceptions.
Sometimes you need to spell out numbers over 20. If your sentence starts with a number, it must be expressed in words regardless of the number. This same rule applies for numbers that begin a title or heading within the text. Commonly-known expressions that use spelled-out numbers should also be written that way in your text, such as "Luther's Ninety-Five Theses." Compound numbers, such as twenty-seven, are hyphenated with no spaces before or after the hyphen.
In order to prevent confusion for your reader, you sometimes need to combine letters and numbers when you have back-to-back numbers in the text. For instance, you might write "twenty-two .22-caliber rifles." Ordinal numbers follow the same rules as other numbers, so those over 20 should typically be written in numerals with the appropriate ending, such as "35th." When writing out numbers of 1,000 or more, use commas to separate each group of three numbers: 3,000,000.
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