Sampling Bias
Sampling bias occurs when the sampling method inherently favors certain types of units. The sample is not a representative miniature of the population; estimates from the sample are systematically biased away from population values.
Real World
Early polls for the 1936 US presidential election were conducted by phone, sampling only wealthy households, systematically excluding poorer voters and producing a wildly incorrect prediction — a classic case of under-coverage bias.
Exam Focus
Distinguish bias from random error: bias is systematic and increasing sample size does not reduce it.
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