1. Where Bias Can Be Introduced

1.2. Information (or Measurement) Bias

This type of bias refers to a systematic error in the measurement of participant characteristics, exposure or treatment outcome, for example:
  • Participants may be wrongly classified as exposed to being at higher risk of disease when they are not. 
  • The disease may be reported as progressing when it is stable.
This might be due to:
  • Inaccuracies in the measurement tool. 
  • Expectations of trial participants – a participant could be more optimistic because they are assigned to the new treatment group .
  • Expectations of the investigators – also called the ‘observer bias’. This occurs when:
    • Investigators are very optimistic about the effect of the new treatment and interpret more favourably any clinical signal.
    • Investigators monitor the adverse effects of the new treatment more carefully than for the standard treatment. 

Blinding  the allocated treatment to the participant and/or the investigators may prevent such bias. Blinding is of special interest when the trial outcome is subjective, like the reduction in pain, or when an experimental treatment is being compared to a placebo. However, while a blinded randomised trial is considered the gold standard of clinical trials, blinding may not always be feasible: 

  • Treatments may cause specific adverse effects that make them easy to identify.

  • Treatments may need different procedures for administration or different treatment schedules.