Some Important Distributions
Discrete Distributions
- $X \sim Bernoulli(p)$
PMF:
\begin{equation}
\nonumber P_X(k)=\left\{
\begin{array}{l l}
p& \quad \text{for } k=1\\
1-p & \quad \text{for } k=0
\end{array} \right.
\end{equation}
CDF:
\begin{equation}
\nonumber F_X(x)=\left\{
\begin{array}{l l}
0 & \quad \text{for } x \lt 0\\
1-p & \quad \text{for } 0 \leq x \lt 1\\
1 & \quad \text{for} 1 \leq x
\end{array} \right.
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=1-p+pe^s
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=1-p+pe^{i\omega}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=p
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=p(1-p)
\end{equation}
- $X \sim Binomial(n,p)$
PMF:
\begin{equation}
\nonumber P_X(k)={n \choose k}p^k(1-p)^{n-k} \quad \text{for } k=0,1,2,\cdots,n
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=(1-p+pe^s)^n
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=(1-p+pe^{i \omega})^n
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=np
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=np(1-p)
\end{equation}
MATLAB:
R = binornd($n$,$p$)
- $X \sim Geometric(p)$
PMF:
\begin{equation}
\nonumber P_X(k)=p(1-p)^{k-1} \quad \text{for } k=1,2,3,...
\end{equation}
CDF:
\begin{equation}
\nonumber F_X(x)=1-(1-p)^{\lfloor x \rfloor} \quad \text{for }x \geq 0
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=\frac{pe^s}{1-(1-p)e^s} \quad \text{for }s \lt - \ln(1-p)
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=\frac{pe^{i \omega}}{1-(1-p)e^{i\omega}}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac{1}{p}
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{1-p}{p^2}
\end{equation}
MATLAB:
R = geornd($p$)+1
- $X \sim Pascal(m,p)$ (Negative Binomial)
PMF:
\begin{equation}
\nonumber P_X(k) = {k-1 \choose m-1} p^{m}(1-p)^{k-m} \quad \text{for } k=m,m+1,m+2,m+3,...
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=\left(\frac{pe^s}{1-(1-p)e^s}\right)^m \quad \textrm{for} \quad s \lt -\log (1-p)
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=\left(\frac{pe^{i \omega}}{1-(1-p)e^{i \omega}}\right)^m
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac{m}{p}
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{m(1-p)}{p^2}
\end{equation}
MATLAB:
R = nbinrnd($m$,$p$)+1
- $X \sim Hypergeometric(b,r,k)$
PMF:
\begin{equation}
\nonumber P_X(x) = \frac{{b \choose x} {r \choose k-x}}{{b+r \choose k}} \quad \text{for }x=\max(0,k-r), \max(0,k-r)+1,..., \min(k,b)
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac{kb}{b+r}
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{kbr}{(b+r)^2} \frac{b+r-k}{b+r-1}
\end{equation}
MATLAB:
R = hygernd($b+r$,$b$,$k$)
- $X \sim Poisson(\lambda)$
PMF:
\begin{equation}
\nonumber P_X(k) = \frac{e^{-\lambda} \lambda^k}{k!} \quad \text{for } k=0,1,2,\cdots
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=e^{\lambda\left(e^s-1\right)}
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=e^{\lambda\left(e^{i\omega}-1\right)}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\lambda
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\lambda
\end{equation}
MATLAB:
R = poissrnd($\lambda$)
Continuous Distributions
- $X \sim Exponential(\lambda)$
PDF:
\begin{equation}
\nonumber f_X(x)=\lambda e^{-\lambda x}, \quad x>0
\end{equation}
CDF:
\begin{equation}
\nonumber F_X(x)=1- e^{-\lambda x}, \quad x>0
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=\left(1-\frac{s}{\lambda}\right)^{-1} \quad \textrm{for} \quad s \lt \lambda
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=\left(1-\frac{i\omega}{\lambda}\right)^{-1}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac1{\lambda}
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac1{\lambda^2}
\end{equation}
MATLAB:
R = exprnd($\mu$), where $\mu=\frac{1}{\lambda}$.
- $X \sim Laplace(\mu,b)$
PDF:
\begin{align}%\label{}
f_X(x) &= \frac{1}{2b} \exp \left( -\frac{|x-\mu|}{b} \right)
=
\left\{\begin{matrix}
\frac{1}{2b} \exp \left( \frac{x-\mu}{b} \right) & \mbox{if }x \lt \mu
\\[8pt]
\frac{1}{2b} \exp \left( -\frac{x-\mu}{b} \right) & \mbox{if }x \geq \mu
\end{matrix}\right.
\end{align}
CDF:
\begin{equation}
F_X(x)=\left\{\begin{matrix}
\frac{1}{2} \exp \left( \frac{x-\mu}{b} \right) & \mbox{if }x \lt \mu
\\[8pt]
1-\frac{1}{2} \exp \left( -\frac{x-\mu}{b} \right) & \mbox{if }x \geq \mu
\end{matrix}\right.
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=\frac{e^{\mu s}}{1-b^2 s^2} \quad \textrm{for} \quad |s| \lt \frac1{b}
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=\frac{e^{\mu i \omega}}{1+b^2 \omega^2}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\mu
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=2b^2
\end{equation}
- $X \sim N(\mu,\sigma^2)$ (Gaussian Distribution)
PDF:
\begin{equation}%\label{}
f_X(x)=\frac{1}{\sigma\sqrt{2\pi}}e^{-\frac{(x-\mu)^2}{2\sigma^2}}
\end{equation}
CDF:
\begin{equation}
F_X(x)=\Phi\left(\frac{x-\mu}{\sigma}\right)
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=e^{\mu s+\frac1{2}\sigma^2s^2}
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=e^{i \mu \omega-\frac1{2}\sigma^2\omega^2}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\mu
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\sigma^2
\end{equation}
MATLAB:
Z = randn, R = normrnd($\mu$,$\sigma$)
- $X \sim Beta(a,b)$
PDF:
\begin{equation}%\label{}
f_X(x)=\frac{\Gamma(a+b)}{\Gamma(a)\Gamma(b)}x^{(a-1)}(1-x)^{(b-1)}, \; \textrm{ for }0 \leq x \leq 1
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=1+\sum_{k=1}^{\infty}\left(\prod_{r=0}^{k-1}
\frac{a+r}{a+b+r}\right)\frac{s^k}{k!}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac{a}{a+b}
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{ab}{(a+b)^2(a+b+1)}
\end{equation}
MATLAB:
R = betarnd($a$,$b$)
- $X \sim \chi^2(n)$ (Chi-squared)
Note:
\begin{equation*}
\chi^2(n) =Gamma\left(\frac{n}{2},\frac{1}{2}\right)
\end{equation*}
PDF:
\begin{equation}
\nonumber f_X(x) = \frac{1}{2^{\frac{n}{2}} \Gamma\left(\frac{n}{2}\right)} x^{\frac{n}{2}-1} e^{-\frac{x}{2}}, \quad \textrm{for } x>0. \end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=(1-2s)^{-\frac{n}{2}} \quad \textrm{for} \quad s \lt \frac1{2}
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=(1-2i\omega)^{-\frac{n}{2}}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=n
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=2n
\end{equation}
MATLAB:
R = chi2rnd($n$)
- $X \sim T(n)$ (The $t$-Distribution)
PDF:
\begin{equation}
\nonumber f_X(x)=\frac{\Gamma(\frac{n+1}{2})}{\sqrt{n\pi}\Gamma\left(\frac{n}{2}\right)}\left(1+\frac{x^2}{n}\right)^{-\frac{n+1}2}
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber \textrm{undefined}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=0
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{n}{n-2} \quad \textrm{for} \quad n>2, \quad \infty \quad \textrm{for } 1 \lt n\leq 2, \quad \textrm{undefined} \quad \textrm{otherwise}
\end{equation}
MATLAB:
R = trnd($n$)
- $X \sim Gamma(\alpha, \lambda)$
PDF:
\begin{equation}
\nonumber f_X(x) = \frac{\lambda^{\alpha} x^{\alpha-1} e^{-\lambda x}}{\Gamma(\alpha)}, \quad x>0
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=\left(1-\frac{s}{\lambda}\right)^{-\alpha} \quad \textrm{for} \quad s \lt \lambda
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac{\alpha}{\lambda}
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{\alpha}{\lambda^2}
\end{equation}
MATLAB:
R = gamrnd($\alpha$,$\lambda$)
- $X \sim Erlang(k, \lambda) \left[= Gamma(k,\lambda)\right]$, $k>0$ is an integer
PDF:
\begin{equation}
\nonumber f_X(x) = \frac{\lambda^{k} x^{k-1} e^{-\lambda x}}{(k-1)!}, \quad x>0
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=\left(1-\frac{s}{\lambda}\right)^{-k} \quad \textrm{for} \quad s \lt \lambda
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac{k}{\lambda}
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{k}{\lambda^2}
\end{equation}
- $X \sim Uniform(a,b)$
PDF:
\begin{equation}
\nonumber f_X(x)= \frac1{b-a}, \quad x \in [a,b]
\end{equation}
CDF:
\begin{equation}
\nonumber F_X(x)=\left\{
\begin{array}{l l}
0& \quad x \lt a\\
\frac{x-a}{b-a} & \quad x \in [a,b)\\
1 & \quad \textrm{for } x\geq b
\end{array} \right.
\end{equation}
Moment Generating Function (MGF):
\begin{equation}
\nonumber M_{X}(s)=\left\{
\begin{array}{l l}
\frac{e^{sb}-e^{sa}}{s(b-a)} & \quad s\neq0\\
1 & \quad s=0
\end{array} \right.
\end{equation}
Characteristic Function:
\begin{equation}
\nonumber \phi_{X}(\omega)=\frac{e^{i\omega b}-e^{i \omega a}}{i\omega(b-a)}
\end{equation}
Expected Value:
\begin{equation}
\nonumber EX=\frac{1}{2}(a+b)
\end{equation}
Variance:
\begin{equation}
\nonumber \textrm{Var}(X)=\frac{1}{12}(b-a)^2
\end{equation}
MATLAB:
U = rand or
R = unifrnd($a$,$b$)
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Practical uncertainty: Useful Ideas in Decision-Making, Risk, Randomness, & AI
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