Koo,

Keep in mind that an event rate is only useful if there is a unit of time. It's actually not very informative to express as events/patient (288 events/2400 patients) without a unit of time. Some patients might just start in trial today. Others have been in trial for 2 years. That's why patient years is a good normalizer. It takes into account not just patient number but duration in trial. Comparing an event rates from one trial at 6 months to another trial at 12 months to another trial at 18 months is an unfair comparison unless these individual event rates are normalized to a time component.

BearDownAZ