Another look at binomial and related distributions exceeding values close to their centre

Abstract

We generalise the known fact that for binomial $X_{n,k} \sim \mathrm{Bin}(n, k/n)$ one has $\inf_{k>1,n} \mathrm{P}(X_{n,k} \geq k) \geq \lim_{k \to 1+}\mathrm{P}(X_{2,k} \geq k) = 1/4$ to cover probabilities of exceeding a constant shift away from the mean. The proof is very short and the theorem yields the original result as a special case, as well as proving that an analogous result holds for the Poisson distribution. We furthermore prove a similar property holds for the family of beta distributions near their mode. Thanks to the connection between Binomial and Beta distributions, this allows us to shed some further light on the original result regarding Binomial probabilities.