Simulation: power analysis for array experiments
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The figure shows a computer simulation experiment.  The data (from Dozmorov, Galecki,  Chang, Krzesicki, Vergara and Miller, "Gene expression profile of long-lived Snell dwarf mice," J. Gerontol. Biol. Sci., in press) reflect a comparison of eight young normal to eight young dwarf mice for expression of 2352 genes in liver.  In the actual experiment (8 mice/group, show at right in figure), we found 72 genes that were called significant using the Tusher "SAM" algorithm, using a false discovery rate criterion of 0.1.  In the simulation, we picked 10 different random sets of 6 mice (or 4, or 3, or 2) from each group, and calculated the number of significant genes; each simulation is shown as a separate symbol in the graphic.  For simulations of six mice/group, for example, individual simulations found as many as 64 or as few as 33 significant gene changes.  For simulations in which each group contained only 3 mice, in contrast, no experiment found more than 30 significant changes.