Incidence vs Prevalence
How to tell Incidence and Prevalence apart when a board question makes them look alike.
Incidence is new cases over time; prevalence is all existing cases and roughly equals incidence multiplied by disease duration when steady-state assumptions hold.
Side by side
Incidence measures new cases arising in a population over a time interval and reflects disease risk.
Prevalence measures all existing cases at a point or period and reflects disease burden and duration.
Incidence measures new cases arising in a population over a time interval and reflects disease risk.
Prevalence measures all existing cases at a point or period and reflects disease burden and duration.
Incidence = new cases / population at risk / time.
Prevalence = existing cases / population; at steady state, prevalence approximates incidence x duration.
The trap
Incidence is new cases over time; prevalence is existing cases.
Watch it explained
Test it nowThe repair move
Before naming the answer, force the stem through the separator: Incidence is new cases over time; prevalence is existing cases.
Try a free question
CoreA population health study measures the number of NEW cases of type 2 diabetes diagnosed in a city of 100,000 over the year 2025. There were 1,200 new diagnoses among 80,000 people initially at risk (not previously diagnosed). What measure does this represent?
Show answer and explanation
Answer: A. Incidence rate. Incidence is the rate of NEW cases arising in a population over a time interval, measured among those AT RISK (not already diseased). Incidence reflects disease risk and incorporates time. Formula: incidence = new cases / (population at risk x time). Useful for understanding disease causation and risk factors.
Why the other choices are tempting
- B. Prevalence counts ALL existing cases (new + old) at a point in time, not just new cases.
- C. Cumulative incidence is the proportion of new cases over a period without rate denominator.
- D. Mortality rate is deaths, not diagnoses.
- E. Standardized morbidity ratio is a comparison measure, not a rate.
Flashcards
Tap a card to flip it.
What clue supports Incidence?
Choose Incidence when the stem gives this pattern: Incidence measures new cases arising in a population over a time interval and reflects disease risk.
What clue supports Prevalence?
Choose Prevalence when the stem gives this contrasting pattern: Prevalence measures all existing cases at a point or period and reflects disease burden and duration.
Quick answers
What is the fastest way to tell Incidence from Prevalence?
Incidence is new cases over time; prevalence is all existing cases and roughly equals incidence multiplied by disease duration when steady-state assumptions hold.
What is the key clue for Incidence?
Incidence measures new cases arising in a population over a time interval and reflects disease risk.
What is the key clue for Prevalence?
Prevalence measures all existing cases at a point or period and reflects disease burden and duration.
What is the common trap with Incidence vs Prevalence?
Incidence is new cases over time; prevalence is existing cases.