1. Onset of exposure: A cancer patient cannot have tumor recurrence until the tumor(s) has been removed. In example 2 mastectomy marked the onset of exposure to the risk of breast cancer recurrence.

2. Randomisation to treatment:In clinical trials the rule of thumb is to take randomisation as the origin. This criterion usually overrides criterion 1. Survival time prior to randomisation is not relevant to a clinical trial. Furthermore, a rule of thumb is "Once randomised, always analyse". If a patient dies between randomisation and start of treatment, time since randomisation should still be analysed.

3. Strongest effect: Time since removal of tumor is usually more related to tumor recurrence than age. So the former is preferred. Time since a baseline community survey tend to be less related to mortality than age. So the latter is often preferred.

Allison PD. Survival Analysis using the SAS^(R) System. Cary, NC: SAS Institute, 1995: 22-25.