What is "poisson distribution"

{k!}

| cdf = $\frac{\Gamma(\lfloor k+1\rfloor, \lambda)}{\lfloor k\rfloor !}$, or $e^{-\lambda} \sum_{i=0}^{\lfloor k\rfloor} \frac{\lambda^i}{i!}\$, or Q(⌊k + 1⌋, λ)

(for k ≥ 0, where Γ(x, y) is the incomplete gamma function, ⌊k⌋ is the floor function, and Q is the regularized gamma function)

| mean = λ
| median =  ≈ ⌊λ + 1/3 − 0.02/λ⌋
| mode = ⌈λ⌉ − 1, ⌊λ⌋
| variance = λ
| skewness = λ
| kurtosis = λ
| entropy = $\lambda[1 - \log(\lambda)] + e^{-\lambda}\sum_{k=0}^\infty \frac{\lambda^k\log(k!)}{k!}$

(for large λ) $\frac{1}{2}\log(2 \pi e \lambda) - \frac{1}{12 \lambda} - \frac{1}{24 \lambda^2} -$
$\qquad \frac{19}{360 \lambda^3} + O\left(\frac{1}{\lambda^4}\right)$

| pgf = exp(λ(z − 1))
| mgf = exp(λ(e − 1))
| char = exp(λ(e − 1))
| fisher = λ

}} In probability theory and statistics, the Poisson distribution (French pronunciation ; in English usually ), named after French mathematician Siméon Denis Poisson, is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since the last event. The Poisson distribution can also be used for the number of events in other specified intervals such as distance, area or volume.

For instance, an individual keeping track of the amount of mail they receive each day may notice that they receive an average number of 4 letters per day. If receiving any particular piece of mail doesn't affect the arrival times of future pieces of mail, i.e., if pieces of mail from a wide range of sources arrive independently of one another, then a reasonable assumption is that the number of pieces of mail received per day obeys a Poisson distribution. Other examples that may follow a Poisson: the number of phone calls received by a call center per hour, the number of decay events per second from a radioactive source, or the number of pedicabs in queue in a particular street in a given hour of a day.