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Psicothema, 2008. Vol. Vol. 20 (nº 4). 933-938




Diseño y análisis de la potencia: n y los intervalos de confianza de las medias

José Fernando García, Juan Pascual, María Dolores Frías, Dirk Van Krunckelsven* y Sergio Murgui**

Universidad de Valencia, * Katholieke Universiteit Leuven (Belgium) y ** Universidad Católica de Valencia

En este estudio se analiza la validez del criterio del 80% de potencia estadística para que no se solapen las medias de los intervalos de confianza (IC). Varias simulaciones indican que la potencia mínima para que los límites de dos medias no se solapen, cuando el IC está en el 95%, es de 0,80; pero cuando el IC está en el 99%, es 0,86; y cuando el IC está en el 90%, es 0,75. Si hay más de dos medias, la potencia mínima aumenta considerablemente. Siendo todavía mayor este aumento cuando las medias poblacionales no aumentan monotónicamente. Por lo tanto, para garantizar que los límites no se solapen, en la mayoría de las situaciones analizadas es necesario calcular directamente el mínimo número de observaciones, siendo de poca utilidad los criterios convencionales de la potencia mínima de 0,80.

Design and power analysis: n and confidence intervals of means. In this study, we analyzed the validity of the conventional 80% power. The minimal sample size and power needed to guarantee non-overlapping (1 - a)% confidence intervals for population means were calculated. Several simulations indicate that the minimal power for two means (m= 2) to have non-overlapping CIs is .80, for (1 - a) set to 95%. The minimal power becomes .86 for 99% CIs and .75 for 90% CIs. When multiple means are considered, the required minimal power increases considerably. This increase is even higher when the population means do not increase monotonically. Therefore, the often adopted criterion of a minimal power equal to .80 is not always adequate. Hence, to guarantee that the limits of the CIs do not overlap, most situations require a direct calculation of the minimum number of observations that should enter in a study.

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