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nihfw epidemiology operation research malnutrition micronutrient deficiency seminar nutrition comprehensive national nutrition survey cnns phases of epidemiological investigation cohort case control study designs stubble burning public health emergency air pollution in north india air pollution in delhi prevention level preventive healthcare web of causation covid-19 surveillance special surveillance sentinel surveillance ihip idsp surveillance basics of epidemiology prevalence incidence descriptive epidemiology uses of epidemiology randomized control studies rct experimental studies community trial clinical trial experimental epidemilogy opertion research on hiv oracle hiv or