Table 8.4.a: A common classification scheme for bias

Type of bias

Description

Relevant domains in the Collaboration’s ‘Risk of bias’ tool

Selection bias.

Systematic differences between baseline characteristics of the groups that are compared.

  • Sequence generation.

  • Allocation concealment.

Performance bias.

Systematic differences between groups in the care that is provided, or in exposure to factors other than the interventions of interest.

  • Blinding of participants and personnel.

  • Other potential threats to validity.

Detection bias.

Systematic differences between groups in how outcomes are determined.

  • Blinding of outcome assessment.

  • Other potential threats to validity.

Attrition bias.

Systematic differences between groups in withdrawals from a study.

  • Incomplete outcome data

Reporting bias.

Systematic differences between reported and unreported findings.

  • Selective outcome reporting (see also Chapter 10).