Questions: Measuring Disease Frequency: Incidence and Prevalence
3 questions to test your understanding
Score: 0 / 3
Question 1 Multiple Choice
A chronic disease has an incidence rate of 50 cases per 1,000 person-years and a mean disease duration of 10 years. What is the approximate steady-state prevalence?
A5 per 1,000
B50 per 1,000
C500 per 1,000
D5,000 per 1,000
The steady-state approximation is prevalence ≈ incidence rate × mean duration. With incidence of 50/1,000 per year and duration of 10 years: (50/1,000) × 10 = 500/1,000. This formula assumes a population in equilibrium where new cases and recoveries/deaths are roughly balanced.
Question 2 True / False
Prevalence is a type of rate because it measures how frequently a disease occurs in a population.
TTrue
FFalse
Answer: False
Prevalence is a proportion, not a rate. A rate requires a time unit in the denominator (e.g., cases per person-year). Point prevalence is the fraction of the population with the disease at a moment in time — it is dimensionless or expressed as cases per population. Calling it a 'prevalence rate' is common but technically incorrect.
Question 3 Short Answer
A new treatment extends the average survival of patients with disease Y from 2 years to 8 years, while incidence remains unchanged. What happens to prevalence and why?
Think about your answer, then reveal below.
Model answer: Prevalence increases approximately fourfold, because prevalence ≈ incidence × mean duration, and duration has quadrupled while incidence is unchanged.
This illustrates that rising prevalence does not always signal more new cases — it can reflect patients living longer with the condition. The distinction matters for policy: rising prevalence from better treatment requires sustained care capacity, while rising incidence requires prevention investment.