In a study to determine the incidence of a chronic disease, 150 people were examined at the end of a three-year period. Twelve cases were found, giving a cumulative risk of 8%. Fifty other members of the initial cohort could not be examined; 20 of these 50 could not be examined because they died. Required:
a. Which source of bias may have affected the study?
b. Does this loss of subjects to follow-up represent a source of bias that may have affected the study results?