This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
All are true of attack rate except 
Select and explain. 
A. It is type of prevalence rate, B. Expressed as percentage, C. Used when population is exposed to risk for a limited period of time, D. Reflects extent of epidemic
A: It is type of prevalence rate
Answer: A: It is type of prevalence rate 
Explanation: Ref:Parks 23rd edition pg 61 Attack rate (case rate) is an incidence rate. Usually expressed in percent Used only when population is exposed to risk for a limited period of time as in an epidemic. Reflects the extent of epidemic. Attack rate = (no. of new cases of a specified disease during a specified time interval / Total population at risk during the same interval) x100