= Sampling bias
<Sampling bias> is a systematic distortion arising when the sampling mechanism makes the observed sample unrepresentative of the target <statistical population> for the estimand of interest. It is distinct from random <sampling variability> and from other causes of the <bias of an estimator>.
Suppose a population has equal-sized groups with values zero and ten, so its mean is five. If <independent> sampling with replacement chooses the first group with <probability> $0.9$, the <sample mean> has expectation one and converges to one as the sample grows. More data reduce its random variation without removing the selection bias. Known selection probabilities can support a design-based correction, but a group never sampled cannot be recovered merely by assigning larger weights to observed units.
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