When do you use a slash mark?

to clarify a relationship in which a hyphenated compound is used.

• the classification/similarity-judgment condition

to separate numerator from denominator

• X/Y

to indicate per to separate units of measurement accompanied by a numerical value

• 0.5 deg/s

to set off English phonemes.

• /o/

to cite an unpublished work.

• Freud (1923/1961)

when a phrase would be clearer

• Each child handed the ball to her mother or guardian.
  not
  Each child handed the ball to her mother/guardian.

for simple comparisons. Use a hyphen or short dash (en dash) instead.

• test–retest reliability
  not
  test/retest reliability

more than once to express compound units. Use centered dots and parentheses as needed to prevent ambiguity.

• nmol • hr-1 • mg -1
  not
  nmol/hr/mg

(adapted from the sixth edition of the APA Publication Manual, © 2010)