Here's a breakdown:
* Spell out: Numbers zero through nine (e.g., "five participants," "two groups").
* Use numerals: Numbers 10 and above (e.g., "12 participants," "25 groups").
Exceptions:
* Numbers beginning a sentence: Always spell out numbers regardless of size. (If this is awkward, restructure the sentence).
* Units of measurement: Always use numerals (e.g., "2 cm," "10 kg," "5 minutes").
* Dates, times, and ages: Always use numerals (e.g., "January 1, 2024," "2:30 p.m.," "12-year-old").
* Statistics: Always use numerals (e.g., "M = 15," "SD = 2.5").
* Fractions: Use numerals for fractions (e.g., "1/2," "3/4"), except when beginning a sentence.
* Decimals: Always use numerals (e.g., "2.5," "0.05").
* Percentages: Always use numerals (e.g., "5%," "25%").
* Money: Always use numerals (e.g., "$10," "€50").
* Page numbers, chapter numbers, etc.: Always use numerals.
* Series of numbers: Use numerals for consistency within a series, even if some numbers are below 10. (e.g., "The participants were given 1, 2, or 3 choices.")
Consistency is paramount. If you use numerals for one number in a sentence (e.g., "10 of the 12 participants"), continue to use numerals for other numbers in the same sentence or paragraph related to the same topic.
When in doubt, consult the Publication Manual of the American Psychological Association, 7th edition, for definitive guidance. The rules are extensive and cover many scenarios not mentioned here.