Bias and variance of estimators

One-line definition

For an estimator $\hat{\theta }$ of a parameter $\theta$, bias is $\mathbb{E}[\hat{\theta }]-\theta$ and variance is $\mathbb{E}[(\hat{\theta }-\mathbb{E}[\hat{\theta }]{)}^2]$. Mean-squared error decomposes as $\mathrm{MSE}={\mathrm{Bias}}^2+\mathrm{Variance}$.

Why it matters

This decomposition is the statistical version of the bias–variance tradeoff familiar from ML: complex models have low bias but high variance; simple models have high bias but low variance. The same accounting applies to any estimator. Sample mean, regularized regression coefficient, importance sampling weight.

The decomposition

For estimator $\hat{\theta }$ of a fixed (non-random) parameter $\theta$:

$$\begin{array}{rl}\mathrm{MSE}(\hat{\theta })& =\mathbb{E}[(\hat{\theta }-\theta {)}^2]\\ & =\mathbb{E}[(\hat{\theta }-\mathbb{E}\hat{\theta }{)}^2]+(\mathbb{E}\hat{\theta }-\theta {)}^2\\ & =\mathrm{Var}(\hat{\theta })+\mathrm{Bias}(\hat{\theta }{)}^2.\end{array}$$

The cross-term vanishes because $\mathbb{E}[\hat{\theta }-\mathbb{E}\hat{\theta }]=0$. Two-line derivation; central to all of statistics.

Why biased estimators can be useful

Unbiased estimators ($\mathbb{E}\hat{\theta }=\theta$) are not always optimal. A biased estimator with much lower variance can have lower MSE.

Examples:

Estimator	Bias	Variance	When better
Sample mean	0	${\sigma }^2/n$	universal
Sample variance with $n-1$	0	larger	unbiased baseline
Sample variance with $n$ (MLE)	small negative	smaller	when minimizing MSE
Ridge regression	nonzero	smaller than OLS	when $X^{\top }X$ is ill-conditioned
Stein estimator	shrinkage bias	strictly lower	always for $\ge 3$ dimensions

The James-Stein estimator (1961) famously dominates the sample mean in $\ge 3$ dimensions despite being biased.

Connection to ML model selection

In supervised learning, the same decomposition holds for the prediction error of a model:

$$\mathbb{E}[(y-\hat{f}(x){)}^2]=\mathrm{Var}(\hat{f}(x))+\mathrm{Bias}(\hat{f}(x){)}^2+{\sigma }_{\epsilon }^2.$$

The ${\sigma }_{\epsilon }^2$ term is irreducible noise. Increasing model capacity decreases bias but increases variance; regularization shifts the tradeoff toward higher bias.

Common pitfalls

• Equating “biased” with “bad.” Many useful estimators are biased; lower MSE is what matters.
• Reporting variance without specifying what’s random. “Variance of the estimator” is over re-sampling the data; “variance of the prediction” is over both data and inputs. Different objects.
• Forgetting that the cross-term vanishes only against the expectation of $\hat{\theta }$. Random fixed offsets ruin the decomposition.
• Confusing estimator variance with model variance. Estimator variance is a property of the estimation procedure; model variance is a property of the model class.