Which of the following is the formula for period prevalence?

Which of the following is the formula for period prevalence?

• A. (# existing cases ÷ population at risk) × 1,000
• B. (# existing cases ÷ population at risk) × 1,000,000
• C. (# new cases ÷ population at risk) × 1,000
• D. (# existing cases ÷ population at risk) × 1,000 or 10,000

Answer: (D)

Period prevalence shows how many people have a condition during a defined time, counting everyone who has the disease at any point in that period. To calculate it, you use all existing cases during the interval (not just new ones) divided by the population at risk, then express the result per a standard number of people, commonly 1,000 or 10,000. That makes the rate easy to interpret and compare across populations. So the correct form is existing cases ÷ population at risk × 1,000 or × 10,000. Using new cases would describe incidence, and 1,000,000 is an uncommon multiplier for period prevalence.

Period prevalence shows how many people have a condition during a defined time, counting everyone who has the disease at any point in that period. To calculate it, you use all existing cases during the interval (not just new ones) divided by the population at risk, then express the result per a standard number of people, commonly 1,000 or 10,000. That makes the rate easy to interpret and compare across populations. So the correct form is existing cases ÷ population at risk × 1,000 or × 10,000. Using new cases would describe incidence, and 1,000,000 is an uncommon multiplier for period prevalence.