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APA Style Rules: Numbers

from: APA Style Lite for College Papers

4.1 Common Numbers (TOC)
Spell out common numbers under 10. "Use figures to express numbers 10 and above and words to express numbers below 10" as long as the numbers below 10 do not express precise measurements and are not grouped with numbers above 10 (APA, 2009, p. 111).

Spell out common fractions, common expressions, and centruries (one-half, Fourth of July, twentieth century).

Spell out all numbers beginning sentences (Thirty days hath September . . .).

To make plurals out of numbers, add s only, with no apostrophe (the 1950s).

When numbers below 10 must be mixed with numbers above 10 in the same sentence they should be written as numerals. For example, write "the students trying out for the soccer team included 5 girls and 16 boys."

Use words and numerals with two numbers in series (five 4-point scales).

Use combinations of numerals and written numbers for large approximate sums (over 3 million people).

Use numerals for numbers 10 and above, for exact statistical references, scores, sample sizes, and sums (multiplied by 3, or 5% of the sample).

Use metric abbreviations with physical measure (4 km) but not when written out (many meters distant).

Use the percent symbol (%) only with figures (5%) not with written numbers (five percent).

Put a leading zero before decimal fractions less than one (e.g., 0.25 km), unless the fraction can never be greater than one, as with statistical probabilities (e.g., p < .01).

Ordinal numbers follow the same rules as other numbers. Spell out ordinals below 10: first, second, . . . ninth. Use numerals for ordinals 10 and above: 10th, 43rd, 99th, and so on. Exception—the twentieth century.

Use numerals for all numbers "that denote a specific place in a numbered series, parts of books and tables, and each number in a list of four or numbers" (APA, 2009, p. 115). Write Grade 6 (but sixth grade); Trial 5; Table 6; page 71 (do not cap page); chapter 8 (do not cap chapter); 2, 4, 6, and 8 words in a series.

Use numerals for all "numbers that represent time; dates; ages; sample, subsample, or population size; specific numbers of subjects or participants in an experiment; scores and points on a scale; exact sums of money; and numerals as numerals" (APA, p. 124). But, spell out approximate days, months, years (new). "She has about fifteen years remaining on her jail sentence."