Inconclusiveness of psychometric testing of medication adherence questionnaires.

Results show that assessing the validity and reliability of adherence questionnaires with standard procedures including statistical tests is inconclusive. Refinement of the constructs of non-adherence is needed in pharmacy and medical practice. We suggest a distinction between the (i) extent of missed doses over the past 2 weeks, (ii) modifiable reasons for non-adherence behavior, and (iii) unmodifiable factors of non-adherence. Validation procedures and corresponding statistical methods should be selected according to the specific single constructs.