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For unknown population: n = z² × p(1−p) / e². For known population: adjust with finite correction. z=1.96 for 95% confidence; e=0.05 for ±5% margin. Use p=0.5 (maximum variance) when proportion is unknown. For a 95% confidence level with ±5% margin: n=385. Doubling precision (±2.5%) quadruples the required sample size. Non-response and sampling bias can invalidate statistically adequate samples.
n ≥ 385