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.