The American Psychological Association, also known as the APA, sets forth a standard or style of writing to which many disciplines, and especially scientific disciplines, conform. The APA style is strongly suited to tackling formatting issues for disciplines that rely heavily on the usage of numbers to make explanations, arguments and deductions from evidence gathered in their respective field. Understanding some of the basic rules governing the use of numbers will help you develop a better sense of how to write in the APA style.
Numerals Below 10
Use written numerals for numbers below 10. For example, five and two makes seven. There is an important exception to this rule though. When you group two numbers for comparison and one of those numbers is below 10 and the other is 10 or above, use Arabic numerals. For example, "8 out of 10 teachers recommend summer reading lists," is the correct way to write this statement.
Numerals 10 & Above
Use Arabic numerals for numbers 10 and above. For example, 10 and 27 makes 37.
The APA format does not use an apostrophe when pluralizing a number. For example, if you were to refer to a decade, it would be correct to use the 1970s, for example, instead of the 1970's.
Fractions & Expressions
Fractions require you to use some judgment. Common fractions such as one-third, one half, and one-fourth should be written out as such. You should write fractions such as 7/32, however, in Arabic numeral form.
Use a combination of Arabic and written numerals for large numbers. It is correct to write "3 million," for example, rather than using the entire Arabic numeral form.
Beginning of Sentences
You should write numbers that begin sentences even if the number is above 10. "Seventeen cars were involved in the crash" is correct whereas "17 cars were involved in the crash" is not. Note, however, that APA discourages writers from using numbers to begin sentences unless you absolutely must.
The same basic rules apply for ordinal numbers. Spell out numbers below 10. "Fourth" is correct whereas "4th" is incorrect. You should write ordinal numbers 10 and above in Arabic numeral form. "23rd" is correct whereas "twenty-third" is incorrect.