Available data evaluating the barriers to the prescription of warfarin in patients with AF have several important limitations. With the exception of 1 survey,34 all evaluations have used case vignettes, which have several methodological limitations.15,28- 33 First, vignettes attempt, but often are not successful in, mimicking a clinical scenario for the physician. Their use rests on the assumption that responses in hypothetical situations reflect actual clinical practice patterns. Comparative data indicate that physician responses to vignettes may not be representative of their clinical practice patterns, and thus this type of analysis may not identify the true barriers present in clinical practice.15,46- 53 In fact, only a few studies (11 of 74) using written case simulations assess validity.47 Attempts at validating vignettes have not been successful,47,49,50 and reliability testing has produced disappointing results.49 Second, the identification of prescribing barriers is inferred or measured indirectly from vignettes. On interpreting the decision to prescribe anticoagulation therapy, assumptions must be made regarding factors triggering the physician's decision. These assumptions may be factors in the vignette, eg, patient age or type of AF. However, there may be circumstances outside of the case that may influence this decision (eg, health care system factors). This information cannot be captured in this format. Third, use of the vignette format does not allow for the assessment of the relative importance of each of the barriers in prescribing warfarin. For example, several factors required when using warfarin (eg, arrangement of laboratory testing, dosage modification) may be identified as barriers to prescribing warfarin, but may not be inconvenient enough to prevent prescription. To identify this information, a rating scale of barrier importance directly eliciting physician responses would be required. Finally, no single study has evaluated the complete spectrum of prescribing barriers for anticoagulation therapy. In general, instruments typically assess patient-related barriers while not assessing physician- and health care system–related barriers. In the only study not using case vignettes, only a partial list of possible barriers is assessed.34 As a result of variability in content in any given survey, certain barriers may be recognized only because they were highlighted, whereas the same barrier may have gone unrecognized in another survey.