Answer (a) — correct: Misclassification may be a problem because it is difficult for subjects to accurately recall what pesticides they may have been exposed to over the course of their entire lives, resulting in incorrect exposure classification. If misclassification of exposure is random, or non-differential between the cases and controls (i.e., not associated with disease status), then its effect will be to bias the estimate towards the null effect of 1.0. However, if misclassification of exposure is related to the outcome or differential between the cases and controls (e.g., cases more accurately recall their exposure experience because they are trying to figure out what could have possible caused their diagnosis), its effect could either increase or decrease estimates of effect.

Answer (b) — 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, or that sick people are more likely to volunteer for a study than healthy people. Regardless, the problem here is not with selection of cases and controls but with difficulty in obtaining accurate information on exposures in the past.

Answer (c) — incorrect: Surveillance bias is a type of selection bias that pertains to disease ascertainment (Aschengrau & Seage, p. 267). The scenario in this situation pertains to exposure ascertainment.