Question 26

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.

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ollege Answer

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.


External validity: the extent to which the study results can be generalised to the greater population, which is influenced by a vast array of factors:

  • The setting and the population from which the sample was selected
  • The inclusion and exclusion criteria
  • The "randomness" of the sample, and the baseline chacteristics of the patients
  • The difference between the trial control group and the routine practice
  • The changes in practice since the publication of the trial
  • The use of patient centered outcomes
  • The degree to which the surrogate outcome measures are related to patient-centered outcomes

Bias: a systematic error which distorts study findings

  • It is caused by flaws in study design, data collection or analysis
  • It is not altered by sample size (increasing sample size only decreases random variations and the influence of chance)
  • It can creep in at any stage in research, from the literature search to publishing of the results.

Selection bias: The selection of specific patients which results in a sample group which is not random, and which is not representative of a population. This can be avoided by randomisation, blinding and by allocation concealment.

The college answer actually comes from the CONSORT Statement glossary:

"Systematic error in creating intervention groups, such that they differ with respect to prognosis. That is, the groups differ in measured or unmeasured baseline characteristics because of the way participants were selected or assigned. Also used to mean that the participants are not representative of the population of all possible participants."


Higgins, Julian PT, and Sally Green, eds. Cochrane handbook for systematic reviews of interventions. Vol. 5. Chichester: Wiley-Blackwell, 2008.