When is it wrong and/or inappropriate to use a slash mark?

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)