PERFORMANCE BIAS:
A bias that arises due to differences in care provided to members of different study groups, other than the treatment/interventions under investigation1,2. Performance Bias may result in Confounding and is typically related to experimental studies (e.g. Randomized Controlled Trials etc.), although it could affect observational studies as well which investigate the effectiveness of a new treatments or interventions.
If patients or clinicians are aware of which groups patients are assigned to, then they may use other forms of treatment in addition to the one prescribed, which may prevent investigators from seeing whether the treatment is truly effective. For example, if a new diet is being tested to see if it leads to weight loss, patients in the diet group may choose to exercise more because they know weight loss is being measured (which also affects weight loss).
Performance Bias may also be observed if the treatment/intervention a patient received ends up being substantially different than what was intended1. Arguably, blinding of patients and investigators to group assignment may be a way to avoid Performance Bias. If blinding is not possible, then using other strategies to control Confounding may be necessary. Also see: Confounding Bias, Detection Bias, Halo Bias, Contamination Bias, Compliance Bias, and Mistaken Identity Bias.
References:
1. Porta M, ed. A Dictionary of Epidemiology. Sixth ed. New York, NY: Oxford University Press 2014. (Link to Reference)
2. McCambridge J, Sorhaindo A, Quirk A, Nanchahal K. Patient preferences and performance bias in a weight loss trial with a usual care arm. Patient Educ Couns. 2014;95(2):243-7. (Link to Reference)