Evidence-Based Medicine

There are many types of biases in research that have been categorized. They fall mainly into three groups with subcategories in each. These are listed below, with an explanation of the more common types. Remember, most biases are unintentional, but we are all human and subject to the often unconscious sway of money, despite best intentions and admonitions that money will not influence doctors’ practice or research. Common sense, sociological research, and many real life examples unfortunately belie these admonitions. AME has no objection to doctors receiving income from companies for work, and believes that such funded research can be valuable. However, AME also believes that this information should be revealed to the patient so it can be taken into account.

I. Selection Bias

  • Volunteer or referral bias: Volunteer or referral bias occurs because people who volunteer to participate in a study (or who are referred to it) are often different than non-volunteers/non-referrals.
  • Non-respondent bias: Non-respondent bias occurs when those who do not respond to a survey differ from those who respond.

II. Measurement Bias

  • Instrument bias
  • Insensitive measurement bias
  • Expectation Bias: Expectation bias occurs in the absence of masking or blinding, when observers may err in measuring data toward the expected outcome. This bias usually favors the treatment group.
  • Attention bias: People who are part of a study are usually aware of their involvement, and as a result of the attention received may give more favorable responses.
  • Verification or work-up bias

III. Intervention (Exposure) Bias

  • Contamination bias
  • Co-intervention bias
  • Timing bias(es)
  • Compliance bias
  • Withdrawal bias: Withdrawal bias occurs when subjects who leave the study (drop-outs) differ significantly from those that remain.
  • Proficiency bias