A journalist reports that patients seen in Outpatient Clinic A were twice as likely to die from MI than patients seen in Outpatient Clinic B over a five-year period. The reported result may be an erroneous conclusion because:

a) It may not have considered the types of patients treated at each clinic
b) It may have been calculated as an efficacy analysis
c) It may not have adjusted for loss-to-follow-up cases
d) It may have been calculated as an intention-to-treat analysis