Poisson Distribution

Understanding Poisson Distribution

Poisson Distribution is a statistical probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time or space. These events must occur with a known constant mean rate and independently of the time since the last event. The Poisson Distribution is named after French mathematician Siméon Denis Poisson.

Characteristics of Poisson Distribution

The Poisson Distribution is characterized by the parameter λ (lambda), which is the average number of events in the given interval. It is a discrete distribution, meaning that it predicts the probability of a number of events happening in a fixed period of time. It is used for independent events which occur at a constant rate within a given continuous medium.

Some of the key characteristics of the Poisson Distribution include:

  • The number of successes in two disjoint time intervals is independent.
  • The probability of a success during a small time interval is proportional to the entire length of the time interval.
  • The probability of more than one success in such an interval is negligible.

Mathematical Formula of Poisson Distribution

The probability mass function (PMF) of the Poisson Distribution for finding exactly k events in an interval is given by:

P(X = k) = (λ^k * e^(-λ)) / k!


  • X is a Poisson random variable

  • e is the base of the natural logarithm (e ≈ 2.71828)
  • k is the number of occurrences of an event — the probability of which is given by the function
  • k! is the factorial of k
  • λ is the average number of events per interval

Applications of Poisson Distribution

Poisson Distribution is used in various fields to model random events. Some common examples include:

  • Modeling traffic flow and the number of car accidents
  • Estimating the number of calls received by a call center
  • Predicting the number of natural occurrences, such as earthquakes or floods
  • Calculating the number of decay events from a radioactive source in nuclear physics
  • Assessing the number of printing errors in a book

Assumptions and Limitations

For the Poisson Distribution to be an accurate model, certain conditions must be met:

  • Events are independent of each other.
  • The average rate at which events occur is constant.
  • Two events cannot occur at the same instant.

However, the Poisson Distribution has limitations. It may not be a good model if the events are not independent or if the rate at which events occur is not constant. Additionally, it is not suitable for events that can happen more than once at the same time.

Relationship with Other Distributions

The Poisson Distribution is related to other probability distributions. For instance, if the number of events in a time period follows a Poisson Distribution, then the time between each event follows an Exponential Distribution. Moreover, under certain conditions, the Poisson Distribution can be used as an approximation for the Binomial Distribution when the number of trials is large, and the probability of success is small.


The Poisson Distribution is a powerful tool for modeling the probability of events occurring within a fixed period. Its simplicity and the fact that it only requires one parameter (λ) make it a popular choice for probabilistic modeling in various fields. Understanding its assumptions and limitations is crucial for its appropriate application to ensure accurate and meaningful results.

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