Probability and Statistics

Mean
$\bar{x} = \dfrac{\Sigma x}{n}$   or   $\bar{x} = \dfrac{\Sigma xf}{\Sigma f}$
 

Variance
$\sigma^2 = \sum \dfrac{(x - \bar{x})^2}{n}$   or   $\sigma^2 = \dfrac{\Sigma (x - \bar{x})^2 f}{\Sigma f}$
 

Permutation
$^{n}P_k = \dfrac{n!}{(n - k)!}$
 

Combination
$\displaystyle ^{n}C_k = \binom{n}{k} = \dfrac{n!}{k! \, (n - k)!}$
 

Binomial Distribution
$P(x) = {^{n}C}_x \, p^x q^{n - x}$
 

Poisson Distribution
$P(x) = \dfrac{e^{-\mu} \mu^x}{x!}$