Set 1 - Overview
Compound words may be written as:
(a) two separate words (e.g., least squares solution; children of the same sex)
(b) a hyphenated word (e.g., same-sex children), or
(c) one unbroken "solid" word (e.g., interstimulus interval)
Check the dictionary to established usage of permanent compounds (e.g., high school, self-esteem, database).
The APA follows Webster's Collegiate dictionary.