What is 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 [[Random|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 This means that the sample may no longer represent the population to be analyzed.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 [[Ethics|ethical]] concerns, or because there are [[Extreme value|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}} [[Category:Statistics]] [[Category:Bias]]