Expanding the evidence base in transplantation: toward a balanced approach.

The use of a one-dimensional "hierarchy of study designs" to grade the evidence on the effectiveness of healthcare interventions is problematic as it may underestimate the limitations of randomized controlled trials and overstate those of observational studies. Observational studies can be an alternative to randomized controlled trials or a source of complementary information. The role of observational studies will vary in different situations. Depending on their topic, design, and methods of analysis, observational studies may be as credible as randomized studies.