Sample Size for Mean
Required n for a target margin of error.
Inputs
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Formula
n = (z* σ / E)²
Plans sample size given desired margin E, assumed σ, and confidence level via z critical value.
Worked example
To estimate a mean within ±2 with σ≈15 at 95% needs n ≈ 217.
Where this can give the wrong answer
- Round up — fractional people impossible.
- σ guess wrong wastes money or precision.
- Finite population correction omitted.
FAQ
- Pilot study or literature estimate — sensitivity analysis with high/low σ.