Let $X \sim \text{Binom}(n, p)$ a binomial random variable. I want to show that : $$\forall 0 < t < 0.9, \quad \exists C, \quad \forall n >1, \quad \mathbb P\bigg(|X-np| \leq C\sqrt{n}\bigg) \geq t$$

I want to prove the result for all $n$, so pure asymptotic is not enough.

**1-** Using the normal approximation together with a Berry-Essen bound we can prove that : $$\mathbb P\bigg(|X-np| \leq x\sqrt{p(1-p)}\sqrt{n}\bigg) > \Phi(x) - \Phi(-x) - \frac{2C(p^2 + (1-p)^2)}{\sqrt{np(1-p)}}
$$
Taking $x = 3$ and $C = .4215$, this yield (part of) the result when $np(1-p)$ is not too small, for instance $np(1-p) \geq 1$ and $n \geq 1000$. Remains to prove for $n < 1000$ and small $p$.

**2-** When $np$ is small, we have a great bound on the absolute error for the Poisson approximation : $$\bigg| \text{Binom}(n, p) - \text{Pois}(np) \bigg| \leq p(1 - e^{-np})$$

But I can't show the result for a Poisson random variable.

Any help, either with the Poisson r.v. or using another approach would be greatly appreciated.

Footnotes

- The value for $C$ comes from Nagaev, Chebotarev (2010).
- I said $t < 0.9$ but anything that generalizes to $t < 0.99$ and beyond is great, obviously we can't go as far as $t = 1$.