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
[{'content': 'Study design of choice for finding out Diurnal variation of fat in Expressed breast milk (EBM) for preterm babies \nSelect and explain. \nA. Prospective coho, B. Ambispective coho, C. Case control, D. Cross sectional', 'role': 'user'}, {'content': "Answer: D \nExplanation: Diurnal variation- fat in Expressed Breast Milk- more in the evening as compared to the morning. To perform this study, we only need to take 2-3 samples in 1-2 days in an adequate sample size. Since this study is being performed at a paicular point in time, it is a CROSS SECTIONAL study. - CROSS SECTIONAL STUDY: Done at point of time, neither forward nor backward. Can't calculate strength of association. Gives prevalence Based on primary data . - Ambispective coho study: observations are bidirectional (past and future both) from the time point it begins.", 'role': 'assistant'}]