Biaix (estadística)

En estadística, s'anomena biaix d'un estimador a la diferència entre la seva esperança matemàtica i el valor del paràmetre que estima. Un estimador amb un biaix nul es diu no esbiaixat, centrat o de tendència central.

En notació matemàtica, donada una mostra $x_{1},\dots ,x_{n}$ i un estimador $T(x_{1},\dots ,x_{n})$ del paràmetre mostral $\theta$ , el biaix és

\ E(T)-\theta

El no tenir biaix és una propietat desitjable en els estimadors. Una propietat relacionada amb aquesta és la de la consistència: un estimador pot tenir un biaix però la mida d'aquest pot convergir a zero conforme creix la grandària mostral.

Donada la importància de la falta de biaix, de vegades, en lloc d'estimadors naturals se n'utilitzen d'altres corregits per eliminar el biaix. Així passa, per exemple, amb la variància mostral.

Fonts del biaix en les ciències experimentals

En el disseny i elaboració d'un estudi d'investigació en clínica, hi pot haver diferents tipus de biaixos:

de selecció: pel fet que els grups no comparables per raó de com es van triar els pacients o subjectes.
d'informació: pel fet que els grups no comparables per raó de com es van obtenir les dades.
de confusió: a causa d'una barreja d'efectes per raó d'una tercera variable (variable de confusió).

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