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short communications
In using normal probability plots for comparing two sets of crystallographic data [Abrahams, S. C. & Keve, E. T. (1971), Acta Cryst. A27, 157] note should be taken of the fact that the expected values of normal order statistics are not given exactly by the percentage points of the normal distribution. This becomes an important consideration only for small samples. Tables of expected ranked exact moduli of normal observations, for sample sizes to 41, are presented: these are useful for half-normal probability plots.