MSFinals-0586

A clinical trial was conducted to compare operative treatment with bracing therapy for scoliosis. Thirty patients who met the criteria for surgery were assigned to group A, while 60 patients who refused surgery or had contraindications were treated with bracing alone in group B. The patients in group A were older and had a poorer performance status compared to group B. Three patients in each group were lost to follow-up for unknown reasons. What is the most probable bias in this study?