For determining the distribution of a set of numbers, the guideline stating that if the distribution has only one mode, if the mode is the same as the arithmetic mean, and if the frequencies decline continuously on both sides of the mode, then more than 1 - (1/2.25t²) of any distribution will fall within the closed range X̄ ± tσ, where t = number of items in a set, X̄ = average, and σ = standard deviation.