Do not hyphenate -ly and superlative words (widely used test, best informed students).
Do not hyphenate common prefixes (posttest, prewar, multiphase, nonsignificant) unless needed for clarity (pre-existing).
Do not hyphenate foreign, letter, numeral terms (a priori hypothesis, Type A behavior) when the meaning is clear without it (least squares solution, heart rate scores).
Do not hyphenate if a noun comes first (a therapy was client centered, results of t tests).
Hyphenate adjectival phrases (role-playing technique, high-anxiety group, two-way analysis).
Hyphenate compound adjectives preceding nouns (client-centered therapy, t-test scores) unless the compound adjective involves a superlative (best written paper).
Hyphenate if the base is an abbreviation or compounded (pre-UCS, non-college bound).
Hyphenate if the base word is capitalized or a number (pre-Freudian, post-1960).
Hyphenate if the words could be misunderstood without a hyphen (re-pair, un-ionized, co-worker).
If in doubt, consult a recently published dictionary. Standards change. For example, "data base" is now "database," and "life-style" is now "lifestyle."
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