Describes a new on-line statistical tool for calculating power in simple experimental designs.\ud \ud Power is an important concept in statistics, if only for the very practical reason that many grantgiving bodies now require a minimum power of 80 per cent to be built into the design of any study eligible for funding. At the same time, though the idea is essentially quite simple, standard textbooks often make the calculations appear mysterious. The provision of computer programs or tables to enable the necessary calculations, though functional, provides no insight into what is going on. The development of a visual aid or ‘nomogram’ by Douglas Altman was a major step forward, making the calculation – at least for a typical independent groups scenario – almost instantaneous. But Altman’s version used only two alpha levels (both 2-tailed), and given the design, the use of even two levels made the diagram somewhat overcrowded. Moreover, it provided no insights into the underlying principles on which it operated
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