Ans. is 'a' i.e., different rates of admission to the hospital Bias is any systematic error in the determination of the association between the exposure and diseaseMany varieties of bias are seen: a) Bias due to confounding : This bias arises because of the confounding factors."A confounding factor is defined as one which is associated with both exposure and disease and is distributed unequally in study and control groups *. More specifically a confounding factor is one that although associated with exposure under investigation is itself independent of any such associations, a risk factor for the disease." * This bias can be removed by matching in case control study.Memory or recall bias : When cases and controls are asked questions about their past history, it may be for the cases to recall the existence of certain events or factors than the controls who are healthy personsSelection Bias: The cases and controls may not be representative of cases and controls in the general population.Interviewer bias: Bias may also occur when the interviewer knows the hypothesis and also knows who the cases are. This prior information may lead him to question the cases more thoroughly than controls regarding a positive history of the suspected causal factor. This type of bias can be eliminated by double blinding.Lead time bias: This leads to an apparent increase in survival among patients whose disease is detected by screening. For example 5 year survival of cancer patients identified on screening might appear to be prolonged simply as a result of starting to count the survival time earlier in the course of disease.Berksonian Bias: It's a special example of bias termed after Dr. Joseph Berkson who recognized this problem. The bias arises because of the different rates of admission to hospitals for people with different diseases.