This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
A person wants to study deafness and mobile phone usage. He collected data on mobile phone usage from government source and deafness from district and private clinics. This type of study is known as: 
Select and explain. 
A. Experimental study, B. Ecological study, C. Etiological study, D. Cross-sectional study
B: Ecological study
Answer: B: Ecological study 
Explanation: Ecological study or correlation study is an analytical study. Here the unit of study is population. Results are not applicable on individual person. It is inferior to case control and coho study. Ref: Park's textbook of Preventive and Social Medicine, 21stedition, page-59.