With reference to clinical studies:
- a) Define the term “external validity”.
- b) Define the term “bias”.
- c) Briefly explain selection bias and measures to reduce it.
Answer and interpretation
a) External validity is the extent to which the results of a study can be generalised to other situations, e.g. different case-mix
b) Bias in statistics is defined as systematic distortion of the observed result away from the “truth”, caused by inadequacies in the design, conduct, or analysis of a trial.
c) Selection bias is caused by a systematic error in creating intervention groups, such that they differ with respect to prognosis. The study groups differ in measured or unmeasured baseline characteristics because of the way participants were selected or assigned. Selection bias also means that the study population does not reflect a representative sample of the target population. Selection bias undermines the external validity of the study and the conclusions drawn by the study should not be extended to other patients.
Measures to reduce selection bias include:
- Randomisation: Randomisation assigns patients to treatment arms by chance, avoiding any systematic imbalance in characteristics between patients receiving experimental versus the control intervention.
- Allocation concealment: The allocation sequence is the order in which participants are to be allocated to treatment. Allocation concealment involves not disclosing to patients and those involved in recruiting trial participants, the allocation sequence before random allocation occurs.
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.