Practicing evidence-based psychiatry: 1. Applying a study's findings: The threats to validity approach.

One critical skill in the practice of evidence-based medicine is the ability to correctly interpret and apply findings of a study relevant to the clinical question needing to be answered. In this paper, we outline an approach to asking the clinical question in a form answerable by a research study and then finding the most relevant study. We next define the two steps in the process of applying a study's findings towards making a treatment decision and discuss the threats to validity approach. We summarize concepts of statistical conclusion validity, internal validity, construct validity, and external validity. We illustrate the method utilizing a clinical vignette that requires choice of an antipsychotic agent. We discuss the results of Clinical Antipsychotic Trial of Intervention Effectiveness (CATIE) in schizophrenia and consider the threats to the different forms of validity in CATIE.