Which multiplier is used in expressing the incidence rate per population?

• A. 1000
• B. 10
• C. 100
• D. 1

Answer: (A)

The main idea is that an incidence rate is a count of new cases divided by the population at risk, and then scaled by a multiplier so the result is easy to read and compare. A common multiplier is 1000, so we express the rate per 1,000 people. This keeps the numbers manageable when incidence isn’t very large. For example, 25 new cases in a population of 20,000 in a year would be (25/20,000) × 1,000 = 1.25 per 1,000 people. Using 1 or 10 would produce tiny, awkward numbers, while 100 would inflate the rate unnecessarily. Sometimes per 100,000 is used for rarer diseases, but in many practice contexts per 1,000 is the standard. So the multiplier used is 1000, giving the rate per 1,000 population.