Answer (a) — incorrect: Selection bias refers to the way cases and controls were selected. In this situation the problem is not with selection but with the collection of exposure information.

Answer (b) — correct: Information bias occurs when the means of obtaining information from cases and controls differ. This is potentially problematic since it suggests that there may be differences in the way in which exposures are reported and classified between the cases and controls (rather than actual exposure status). It could be that study investigators spent more time with cases to ensure that all exposures were correctly classified. If they interviewed the controls less thoroughly, misclassification may be greater in the controls than in the cases, i.e. misclassification tied to disease status. However, having a longer average interview for cases does not automatically imply that there is information bias, especially if the reason for the differing interview times has to do with the fact that cases reported more exposures than controls due to either the exposure having a true effect on case/control status or if cases were of poor health and needed more time to answer the questions.

Answer (c) — incorrect: Volunteer bias is a type of selection bias resulting from the tendency for one's health to influence his or her decision to participate in a study. One could argue either that healthy people are more likely to volunteer for a study than sick people because they are concerned about their health, or that sick people are more likely to volunteer for a study because they worry about the disease and are seeking a diagnosis, care, etc. Regardless, here we deal not with selection of cases and controls but with differential interviewing methods.

Answer (d) — incorrect: Loss to follow-up is a type of selection bias that pertains mostly to cohort studies. Differential loss to follow-up can bias the results of a cohort study when study subjects who can no longer be located or who no longer want to participate are more or less likely to be exposed and develop the disease than those subjects who remain in the study (Aschengrau & Seage, pp. 268-270).