Which rate is most useful in studying chronic diseases and includes both old and new cases in a given period?

• A. Prevalence rate
• B. Incidence rate
• C. Mortality rate
• D. Case-fatality rate

Answer: (A)

When studying chronic diseases, you want a measure that reflects how many people are living with the condition at a given time, not just how many are newly diagnosed. Prevalence does exactly that: it tells you the number of existing cases in the population during a period (or at a point in time) divided by the population size. Because chronic diseases persist for long periods, many people accumulate as cases over time, so the prevalence captures both those who have had the disease for a while and those who were newly diagnosed. This makes it the best indicator for understanding overall disease burden, planning healthcare services, and allocating resources.

Incidence, on the other hand, counts only new cases, which is useful for studying risk and the development of disease but doesn’t convey how many people are currently living with the condition. Mortality rate measures deaths in the population, not how many people are living with the disease. Case-fatality rate looks at the proportion of diagnosed cases that result in death, which doesn't describe how widespread the disease is in the community.