Selection bias

Selection bias is a kind of bias that is introduced by the selection of individuals, groups or data for analysis in such a way that proper randomization is not achieved. This means that the sample may no longer represent the population to be analyzed. Sometimes, this is called selection effect.

There are different ways a selection bias can be introduced, for example.

There is a systematic error because the samples of the population are not random.
The trial is ended early, either because the desired conclusion can be reached, or because there are ethical concerns, or because there are extreme values that are hard to explain or conpensate.

A statistical method called Heckman correction can correct some of these, in certain circumstances.

References

James Heckman (1979): Sample selection bias as a specification error. Econometrica, 1979, pp.. 153–161.

Geddes, Barbara: How the Cases You Choose Affect the Answers You Get: Selection Bias in Comparative Politics, in: Political Analysis 2 (1990), 1, pp. 131–150.

Biases
Cognitive biases	Attribution Correspondence Actor–observer Dunning–Kruger Halo effect Overton window Acquiescence Ambiguity Anchoring Attentional Authority Automation Availability Mean world Belief Blind spot Choice-supportive Commitment Confirmation Compassion fade Congruence Courtesy Cultural Distinction Egocentric Curse of knowledge Emotional Extrinsic incentives Fading affect Framing Frequency Hindsight Horn effect Hostile attribution Impact Implicit In-group Mere-exposure effect Negativity Normalcy Omission Optimism Out-group homogeneity Outcome Precision Present Pro-innovation Response Restraint Self-serving Social comparison Social influence bias Status quo Substitution Time-saving Trait ascription Turkey illusion von Restorff effect Zero-risk In animals
Statistical biases	Observer Selection Estimator Forecast Healthy user Information Psychological Lead time Length time Non-response Omitted-variable Participation Recall Sampling Self-selection Social desirability Spectrum Survivorship Systematic error Systemic Verification Wet
Other biases	Media False balance Vietnam War Norway South Asia Sweden United States Arab–Israeli conflict Ukraine Academic Funding FUTON Inductive Infrastructure Inherent In education Net Political bias Publication Reporting White hat
Bias reduction	Cognitive bias mitigation Debiasing Heuristics in judgment and decision-making
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