Identify which of the following cases would violate the assumptions of the Kaplan–Meier method.

a. Patients are more likely to leave the study before the end of monitoring if they are relatively healthy than if they are relatively unhealthy.

b. Patients who enter the study at an early calendar date are in worse health on average than patients who enter the study at a later date.

c. Probability of survival is higher at a later time point in the monitoring period than at an earlier time point.

d. In a study involving several years of patient monitoring, patients entering the study and beginning the monitoring period in winter are more likely to die than patients entering the study and beginning monitoring in summer.