Performance bias is specific to differences that occur due to knowledge of intervention allocation, in either the researcher or the participant. This results in differences in the care received by the intervention and control groups in a trial other than the intervention that is being studied. For example, participants in the control group might seek other treatments, or researchers/clinicians might treat participants differently depending on which group they are in. This bias may overinflate the effect of the intervention, particularly in trials with subjective outcomes.
Performance bias often occurs in trials where it is not possible to blind participants and/or researchers, such as trials of surgical interventions, nutrition or exercise. For example, a systematic review of trials of physical activity for women with breast cancer after adjuvant therapy found that all the included trials were at high risk of performance bias because the nature of the intervention (i.e. physical activity) made it impossible to blind trial personnel and participants, and particularly due to the main outcomes being subjective.
A qualitative study assessed the risk of performance bias in a weight loss trial of a novel patient counselling program compared to usual care in general practice. The control group reported being disappointed at having been offered usual care when they had taken part in the trial. Reactions to disappointment involved both movements toward and away from behaviour change. They conclude that disappointment may introduce bias, as they lead the randomized groups to differ in ways other than the intended experimental contrast.
A systematic review assessed the effect of the lack of double blinding (of both participants and researchers) on intervention effects. It concluded that compared to studies with clear double-blinding, studies with a ‘lack of, or unclear double-blinding’ showed inflated effect estimates on an average of 13% (ROR 0.87, 95% CrI 0.79 to 0.96). The review authors also concluded that studies with subject outcome assessment (such as pain) were more prone to the adverse effects of lack of blinding (ROR 0.85, 95% CrI 0.75 to 0.95).
Ideally, participants and researchers should be blinded to the interventions. If blinding is not feasible, the effect of performance bias can be mitigated by using objective outcomes. For instance, a subjective outcome such as patient-reported pain is more likely to be influenced by performance bias than an objective measure, such as hospital admission.
If subjective outcomes are used in a trial, performance bias can be mitigated by blinding the outcome assessor. For instance, in a trial assessing the effect of cognitive behavioural therapy (CBT), the researcher that delivers the intervention (who cannot be blinded), should be different to the researcher that assesses the patient-reported outcomes.