Berkesonian bias is a selection bias which occurs in-

.bias is any systematic error in the determination of the association between the exposure and disease.the relative risk may increase or decreaseas a result of the bias,it reflects some type of non comparability between the study and control groups.berkensonian bias,named after Joseph Berkenson.this bias arises because of the different rates of admission to hospitals for people with different diseases,that is HOSPITAL CASES AND CONTROLS. ref:park&;s textbook,ed 22,pg no 71