Chebyshev's inequality - Wikipedia
https://en.wikipedia.org/wiki/Chebyshev's_inequalityMarkov's inequality states that for any real-valued random variable Y and any positive number a, we have Pr(|Y| > a) ≤ E(|Y|)/a. One way to prove Chebyshev's inequality is to apply Markov's inequality to the random variable Y = (X − μ) with a = (kσ) : It can also be proved directly using conditional expectation: Chebyshev's inequality then follows by dividing by k σ .