Learn in the textbookPrecision, Bias, and Various Biases
DefinitionSelection bias (選択バイアス) is a statistical bias in which a skewed way of choosing the sample leaves it unrepresentative of the population, which distorts the result.
ExplanationFor example, treating a web survey of your own site's members as the trend for the whole country leaves out everyone who is not a member. Random sampling (無作為抽出), which gives everyone in the population an equal chance of being chosen, is the basic way to reduce this skew.
AnalogyIt is like tasting only the top of an unstirred pot and deciding how the whole pot tastes.
Memory hookA famous story tells of studying bullet holes only on military planes that made it back, while overlooking those shot down (survivorship bias, 生存者バイアス).