Five women are followed for the development of deep vein thrombosis while taking an oral contraceptive. Woman A develops it at 12 months, woman B at 24 months, woman C remains thrombosis free for 36 months, woman D is lost to follow up at 18 months, and woman E completes 30 months without events. The incidence density is:

• A 3 per 120 person-months
• B 2 per 100 person-months
• C 2 per 110 person-months
• D 2 per 120 person-months ✓

Explanation

Person-time sums follow-up until the event or censoring: 12 plus 24 plus 36 plus 18 plus 30 equals 120 person-months. Two events occurred, so incidence density is 2 per 120 person-months. Censored observation time before loss to follow up still contributes, which distinguishes incidence density from simple cumulative incidence and makes it the correct measure when follow-up durations differ between participants.

Reference: Gordis Epidemiology, 6th ed.

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