A prospective cohort study was conducted to determine the risk of heart attack among men with varying levels of baldness. Third year residents in dermatology conducted visual baldness assessments at the start of the study (which was before any heart attacks had occurred). Four levels of baldness were coded: none, minimal, moderate, and severe. The follow-up rate was close to 100%. Which of the following types of bias were surely avoided in this study? a. Recall bias of the exposure information.
b. Differential misclassification of the exposure.
c. Non-differential misclassification of the exposure.
d. Selection bias due to follow up.