Under the half-year convention, how much disease-free follow-up does a participant diagnosed during year 3 contribute?

1. A participant who reaches the end of year 2 disease-free has contributed 2 full person-years. 2. Onset during year 3 could fall anywhere in that year, so on average the participant is treated as disease-free for half of it. 3. That adds 0.5 person-years, giving 2 plus 0.5, or 2.5 person-years. 4. The same half-year rule applies to a participant lost to follow-up in year 3. 5. 3.0 counts the whole of year 3 as disease-free time, which overstates the denominator. 6. 2.0 drops the half year entirely, and 1.5 is what a participant diagnosed during year 2 would contribute. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Definition of incidence rate_