When planning any study, you must balance three constraints: Confidence level — how sure you want to be that the true population value falls within your margin of error (95% is standard for most research). Margin of error (MOE) — acceptable range of uncertainty (±5% is standard). Sample size — larger samples give smaller MOE but cost more to collect. The formula: n = (Z² × p × (1-p)) / e², where Z is the z-score for your confidence level, p is the estimated proportion (0.5 = maximum variance), and e is the desired margin of error.