In a prospective cohort study of smoking and ovarian cancer, women who smoked were less likely to respond to recruitment and enrollment efforts compared with women who did not smoke. The investigators observed a RR=1.4 quantifying the relationship between smoking and ovarian cancer even though a prior retrospective cohort study had found a relationship of RR=2.6. How are the prospective cohort study findings biased compared to retrospective cohort study findings? Biased away from the null Biased toward the null Not biased at all