A final key property of an HRQL measure is Interpretability.
- For discriminating between groups, we could ask whether a particular score signifies that a patient is functioning normally or has mild, moderate, or severe impairment.
- For evaluating change over time, we might ask whether a particular change in a score represents a trivial, small but important, moderate, or large improvement or deterioration.
- A number of strategies are available to make PRO scores interpretable (Guyatt et al., 1991), such as:
- Classify patients into those who had important improvement as well as those who did not and examine the changes in score in the two groups;
- Interpret observed changes in PRO measures in terms of elements of those measures that will be familiar to readers (for instance, descriptions of changes in mobility); or
- Determine how scores in PRO measures relate to marker states that are familiar and meaningful to clinicians.
Data suggest that small, medium, and large effects correspond to changes of approximately 0.5, 1.0, and greater than 1.0 per question for instruments that present response options on seven-point scales (Jaeschke et al., 1989).