Person-Time and Incidence Rate: When Counting People Fails

Person-Time and Incidence Rate: When Counting People Fails is Lesson 2 of 20 in Principles of Epidemiology. People arrive late, leave early and get sick in between, so "how many out of how many" stops working. This lesson teaches person-time, the incidence rate and the reservoir that links incidence to prevalence, with invented Oakhaven figures. This course is taught by Angel Reyes, MPH, MCHES, PMP, PMI-ACP, a doctoral candidate in public health who earned an MPH from a CEPH-accredited program, has planned, managed, and evaluated public health programs, and builds research training for graduate students and practitioners at Subthesis. Part of the 20-lesson course Principles of Epidemiology. Five key facts from this lesson: 1. Risk needs a closed cohort; when people enter late or leave early, the denominator becomes person-time instead of people. 2. In the invented five-person example, 5 + 4 + 3 + 3 + 1 = 16 person-years and 2 cases give an incidence rate of 12.5 per 100 person-years. 3. An incidence rate is a speed of occurrence with units of one over time, so it is not a percentage and is never read as a probability. 4. Multiplying the starting count by the planned years overstates person-time; you add up the time each person was actually observed. 5. In a steady state, prevalence is about the incidence rate times the average duration, so a treatment that prolongs life without cure raises prevalence. Free research tools at https://www.subthesis.com, including the Research Question Builder and the Statistical Power Calculator. Companion courses on this channel: Certified in Public Health: The CPH Exam Pathway, and Foundations of Program Planning. Examples are fictional and all figures are invented for teaching. Oakhaven Health District does not exist. This course is independent educational content from Subthesis and is not clinical, diagnostic, screening, or policy guidance.