APA classifies common abbreviations such as IQ, HIV and REM as words. Any acronym not labeled "abbr" in the 2005 edition of "Merriam-Webster's Collegiate Dictionary" should not be defined in the text. Other abbreviations commonly appear in APA journals, such as RT for reaction time or ITI for intertrial interval. These abbreviations require explanation the first time they are used.
Units of measurement and time are abbreviated when accompanied by numeric values, but spelled out otherwise. Day, week, month and year are exceptions; they are always spelled out. A period follows the abbreviation "in." to prevent confusion. Other measurement abbreviations do not take a period. Abbreviations for organic chemical compounds may be used without explanation if they are listed as words in "Merriam-Webster's Collegiate Dictionary." When a route of administration accompanies a number-and-unit combination, the route can be abbreviated using lower case letters with no period; for example, "iv" for intravenous.
APA style uses standard statistical abbreviations in parentheticals, tables and figures, but spells out the terms within the text. Common statistical symbols include "M" for mean, "SD" for standard deviation and "df" for degrees of freedom. An "N" represents the total number in a sample, but an "n" represents the number in a portion of the sample. Most statistical abbreviations use an italic typeface. However, the symbols for vectors and matrices take a boldface font, and Greek letters and any accompanying superscripts or subscripts use the standard font for the manuscript.
Most Latin abbreviations should only appear in parentheticals. However, "v." is always used for versus in reference to court cases and "et al." always appears when it is needed in citations. Latin abbreviations always contain punctuation, such as in "e.g." for "for example" and "etc." for "and so forth."
Reference lists contain several abbreviations in order to condense the citations. Common abbreviations include "p." or "pp." for page or pages, "n.d." for no date, "Vol." for volume, "ed." for edition, and "Ed." for editor. These abbreviations also appear in parenthetical citations within the main text.