Interpretation of Cohort Study Results: Incidence Rate
The correct interpretation is that 30 people develop breast cancer per 1000 person-years, which represents an incidence rate, not a prevalence or percentage of the population.
Understanding Incidence Rate vs. Prevalence
The finding of "30 per 1000 person-years" is an incidence rate, which measures the rate at which new cases occur over time in a population at risk. This is fundamentally different from prevalence (the proportion of people who have the disease at a given time).
Key Distinctions:
Incidence rate (what the study measured): 30 new cases per 1000 person-years means that if you follow 1000 people for one year, you would expect approximately 30 new breast cancer diagnoses 1, 2
NOT a percentage: This is not stating that 30% of people have breast cancer—that would be epidemiologically impossible and would represent a prevalence far exceeding any known population burden 1
NOT a cumulative risk: This is different from lifetime risk, which for breast cancer is approximately 12% (or 1 in 8 women) over an entire lifetime 1
Context from Established Incidence Data
To understand why option C is correct, consider established breast cancer incidence rates:
European Union: The crude incidence is approximately 94-110 per 100,000 women per year 1
United States: The incidence rate was approximately 124 per 100,000 women in 2003 1
Converting to comparable units: 30 per 1000 person-years = 3,000 per 100,000 person-years, which would be extraordinarily high but represents an incidence rate, not prevalence 1, 2
Common Pitfalls to Avoid:
Confusing incidence with prevalence: Incidence measures new cases over time; prevalence measures existing cases at a point in time 3
Misinterpreting person-years: Person-years accounts for varying follow-up times and represents the total time all participants were observed 2
Converting to percentages incorrectly: An incidence rate cannot be directly converted to a percentage of the population without considering the time dimension 1
The study result should be reported as an incidence rate of 30 per 1000 person-years, which quantifies the risk of developing breast cancer over time in the observed population 1, 3, 2.