Validity -  The ability of a test to measure the characteristic it is supposed to measure.

Values -  Perceptions of what is important in life.

Variable -  An event, characteristic, behavior, or condition that researchers measure and study.

Variable-interval schedule -  A reinforcement schedule in which reinforcement happens after a particular average amount of time.

Variable-ratio schedule -  A reinforcement schedule in which reinforcement happens after a particular average number of responses.

Vestibular system -  The sensory system involved in balance.

Vulnerability-stress model -  The idea that individuals who have a biological vulnerability to a particular disorder will have the disorder only if certain environmental stressors are present.