Probability and Statistics - shivamvats/notes GitHub Wiki

Fundamental Results

  1. Law of Large Numbers: The sample average, i.e. results from performing an experiment repeatedly gets closer to the expected value as the sample size gets larger (perform more experiments). Exceptions: Cauchy distribution (doesn't have an expected value) and Pareto distributions (expectation is infinite).

  2. Central Limit Theorem: The law of large numbers tells us that u -> mu asymptotically, where u is the sample average and mu is the expectation. But how does the error u - mu behave? The Central Limit Theorem says that the error approximately behaves normally:

    sqrt(n)(u - mu) -(in distribution)> N(0, sigma^2)

  3. Gilvenko-Cantelli Theorem: The empirical CDF converges to the true CDF with enough data.

  4. Markov's Inequality: If the mean of a +ve random variable is small, its probability mass can't go too far from it. Pr(X >= t) <= EX/t

  5. Chebyshev's Inequality: Builds on Markov's inequality. The probability of a random variable X deviating by k x std from its mean is upper bounded by``1/k^2``. Pr(|X - mu| >= k x std) <= 1/k^2

  6. Chernoff Method: Give a tighter upper bound than Chebyshev's Inequality.

  7. Gaussian and Sub-Gaussian Tail Bounds: For Gaussians and sub-Gaussians:

    Pr(|X - mu| >= k x std) <= 2 x exp(-k^2/2) or Pr(|X - mu| >= u) <= 2 x exp(-u^2/2(std)^2)

Note that this is much tighter than the more general Chebyshev's inequality.

Distributions

  1. Log-normal Distribution: If x is distributed as a normal distribution, then u = ln(x) is distributed as a log-normal distribution. If u has a distribution A(mu, sigma^2), then y = e^a u^b is distributed as A(a + bmu, b^2 sigma^2).
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