A systematic error that occurs when the sample chosen for a study is not representative of the target population, leading to skewed or invalid conclusions. This bias can happen through non-random sampling methods or when certain groups are more likely to participate than others.
The term emerged from statistics and epidemiology in the early 20th century, combining 'selection' (the process of choosing) with 'bias' (systematic deviation from truth). It gained particular importance as researchers recognized that convenience sampling could severely limit the validity of their findings.
Online surveys suffer from massive selection bias because they automatically exclude people without internet access, computer skills, or time to complete surveys - often the very populations researchers most need to understand! Psychology studies are notorious for being based on 'WEIRD' participants (Western, Educated, Industrialized, Rich, Democratic), making up only 12% of the world's population.
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