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
Study unit of ecological study is: 
Select and explain. 
A. Population, B. Patient, C. Community, D. Case
A: Population
Answer: A: Population 
Explanation: Ans. a. PopulationRef: Park's Text book of Preventive and Social Medicine, 24th Ed; Page No-67There are 2 types of epidemiological studyObservational studyEcological study - population...unit of studyCross sectional study - individual...unit of studyCase-control study - individual...unit of studyChart study - individual...unit of studyExperimental studyRCT/Clinical trials - patients...unit of studyField trial - healthy population...unit of studyCommunity trial - communities...unit of study