Epidemiological terms are top of mind now, due to the pandemic. Here are two that often confuse: incidence and prevalence.
For example, I encountered the following sentence on a popular medical web site:
“Knee meniscal injuries are common with an incidence of 61 cases per 100,000 persons and a prevalence of 12% to 14%.”
I was confused. What was the difference between incidence and prevalence? It turns out the incidence figure needed a time period, without which the statistic was meaningless.
Incidence: New occurrences of a disease or condition over a specified time period. “About 61 cases of meniscal injury occur per year, for every 100,000 people.”
Prevalence: Count of existing cases of the disease or condition at a specific point in time. “About 12%-14% of people suffer from meniscal injury, as of 2021.”
Both are typically expressed as a fraction or percentage of the population.