Unbiased Estimator
A statistic is an unbiased estimator of a parameter if E(statistic) = parameter. For example, the sample mean x̄ is an unbiased estimator of μ because E(x̄) = μ, and the sample variance with (n-1) divisor is an unbiased estimator of σ² because E(s²) = σ².
Real World
The Office for National Statistics divides by (n−1) when calculating variance from its Labour Force Survey samples, ensuring the estimate doesn't systematically understate the true population spread.
Exam Focus
Always explain why dividing by (n−1) rather than n gives an unbiased estimator of the population variance.
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