Final answer:
Criterion-related validity is the statistical measurement that informs a CTRS about the assessment's effectiveness in comparing results to what is supposed to be measured, focusing on external criteria.
Step-by-step explanation:
The statistical measurement that informs a Certified Therapeutic Recreation Specialist (CTRS) about how well the assessment results compare to what is being measured is criterion-related validity. This aspect of validity determines how well one measure predicts an outcome based on criteria external to the measure itself. It's essential in assessing the effectiveness of certain tools and tests in predicting performance or behavior. Criterion-related validity is composed of two types: predictive validity, which determines how well the assessment can predict future performance or behavior; and concurrent validity, which examines how well the assessment correlates with a current measure of the same construct.