Let $X$ be a positive random variable and assume that both $a = EX^{-1}$ and $\mu = EX$ are finite. Define $c^2 = 1 - (a\mu)^{-1}$. This quantity serves as a measure of variability for $X$ which is reflected in the behavior of completely monotone functions of $X$. For $g$ completely monotone with $g(0) < \infty$: $0 \leq Eg(X) - g(EX) \leq c^2g(0) \text{and}\operatorname{Var} g(X) \leq c^2g^2(0).$
@article{1176349668,
author = {Brown, Mark},
title = {A Measure of Variability Based on the Harmonic Mean, and Its Use in Approximations},
journal = {Ann. Statist.},
volume = {13},
number = {1},
year = {1985},
pages = { 1239-1243},
language = {en},
url = {http://dml.mathdoc.fr/item/1176349668}
}
Brown, Mark. A Measure of Variability Based on the Harmonic Mean, and Its Use in Approximations. Ann. Statist., Tome 13 (1985) no. 1, pp. 1239-1243. http://gdmltest.u-ga.fr/item/1176349668/