A prospective cohort study of 400 people was followed for 1 year. Half of the study participants were exposed and half were unexposed.

11a. In the exposed group, 40 participants truly had the disease outcome during the 1-year follow up. What is risk of disease in the exposed group?

11b. In the unexposed group, 20 participants truly had the disease outcome during the 1-year follow up. What is the risk ratio of the disease, comparing those with exposure to those without exposure?

11c. In the study of the cohort described above, 22 of the exposed and 16 of the unexposed were lost to follow-up during the 1-year prospective study. Consequently, only 362 participants and their outcomes were actually observed.

Among the 22 exposed who were lost to follow-up, 11 had the disease after they were lost (unobservable). Among the 16 unexposed who were lost to follow-up, 15 had the disease after they were lost (unobservable). Calculate the observed risk ratio in the cohort of 362.

11d. Compared to the true risk ratio (part 10b), was the observed risk ratio (part 11c) biased toward the null, away from the null, or not biased? Explain your answer in one sentence.