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The coefficient of variation (standard deviation/mean) of the dimensionless number c(δ) plotted as a function of the test exponent δ in Eq. 14. The theoretical optimal value 4/3, as indicated by the dashed line, is close to the empirical minimum (•) reached at δ = 1.36 (±0.06, SD by bootstrap). Data were from table 1 in ref. 11 (27 species).
