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Appraising the evidence: what is measurement bias?

Division of Psychological Medicine, Institute of Psychiatry, King’s College London, Weston Education Centre, London, UK

Correspondence to:
Dr M Henderson, Division of Psychological Medicine, Institute of Psychiatry, King’s College London, 3rd Floor, Weston Education Centre, 10 Cutcombe Road, London SE5 9RJ, UK; m.henderson@iop.kcl.ac.uk

The first 150 words of the full text of this article appear below.

In a previous issue of Evidence Based Mental Health, we discussed the role that selection bias can have in introducing systematic error into studies (see Evid Based Ment Health 2007;10:67–8). In this article we discuss measurement (or information) bias—the other major type of systematic error commonly encountered in epidemiological research (fig 1). Important general points about bias include the following:

• Bias may be introduced by poor study design or poor data collection.
  
• Bias cannot be "controlled for" at the analysis stage.
  
• Bias risks researchers and readers drawing conclusions that are systematically different from the truth.
  
• Bias can lead to an over or underestimation of an effect.
  
• An awareness of how bias may be acting helps a reader interpret study findings more accurately.
  

Measurement bias occurs when information collected for use as a study variable is inaccurate. The incorrectly measured variable can be either a disease outcome or . . . [Full text of this article]