Systematic error or distortion that occurs in the assessment of outcomes due to differences in the care provided to study participants. It occurs when there are variations in the management, administration, or delivery of interventions between the groups being compared in a study. This bias can significantly impact the validity and reliability of research findings in pharmacy practice. For example, if one group of patients in a clinical trial receives medication counseling from highly trained pharmacists while the other group receives minimal guidance, any observed differences in outcomes could be influenced by this disparity in care rather than the effectiveness of the medication itself.