Selection bias

It weakens both the comparison between groups and how far the results apply to real patients. Randomization, clear eligibility criteria and consecutive enrollment are common ways to reduce it.

Bias due to systematic differences between those selected for the study population and those for the intended use population. For example, in a clinical outcome, parallel group study, study results can be subject to selection bias when the investigational and control groups are chosen so that they differ from each other in ways that affect the study outcome.

FDA guidance: Design Considerations for Pivotal Clinical Investigations for Medical Devices (November 2013), Section 11, Glossary