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The Poisson distribution is the discrete probability distribution of the number of events occurring in a given time period, given the average number of times the event occurs over that time period.

A specific fast-food restaurant gets a normal of 3 guests to the drive-through every moment. This is only a normal, be that as it may. The real sum can change. 

A Poisson dispersion can be utilized to dissect the likelihood of different occasions with respect to what number of clients experience the drive-through. It can enable one to figure the likelihood of a respite in movement (when there are 0 clients going to the drive-through) just as the likelihood of a whirlwind of action (when there are at least 5 clients going to the drive-through). This data can, thusly, help a director plan for these occasions with staffing and booking.

Conditions for Poisson Distribution:

An event can occur any number of times during a time period.

Events occur independently. In other words, if an event occurs, it does not affect the probability of another event occurring in the same time period.

The rate of occurrence is constant; that is, the rate does not change based on time.

The probability of an event occurring is proportional to the length of the time period. For example, it should be twice as likely for an event to occur in a 2 hour time period than it is for an event to occur in a 1 hour period.

For example, the Poisson distribution is appropriate for modeling the number of phone calls an office would receive during the noon hour, if they know that they average 4 calls per hour during that time period.

Although the average is 4 calls, they could theoretically get any number of calls during that time period.

The events are effectively independent since there is no reason to expect a caller to affect the chances of another person calling.

The occurrence rate may be assumed to be constant.

It is reasonable to assume that (for example) the probability of getting a call in the first half hour is the same as the probability of getting a call in the final half hour.

Obviously, this circumstance isn’t a flat out ideal hypothetical fit for the Poisson dissemination. For example, the workplace surely can’t get a trillion calls during the timespan, as there are not exactly a trillion people alive to make calls. Essentially, the circumstance is close enough that the Poisson dispersion works admirably of demonstrating the circumstance’s conduct.

Binomial Distribution at Infinity

Consider a binomial distribution of Xsim B(n,p)XB(n,p).

It can be easily shown that P(X=k)={nchoose k}p^k{(1-p)}^{n-k}P(X=k)=(kn)pk(1−p)nk for k=0,1,2,3,ldots,nk=0,1,2,3,…,n.

Now, let’s take the limit of the above using n to inftyn→∞. Instead of having an infinitesimal pp, let’s assume that it is given that npnp, the mean of the probability distribution function, is some finite value mm.

Find P(X=k)P(X=k) in terms of mm and kk for this new distribution, where k=0,1,2,3,ldotsk=0,1,2,3,…, without looking anything up or reciting any formulas from memory.

Probabilities with the Poisson Distribution

Given that a situation follows a Poisson distribution, there is a formula which allows one to calculate the probability of observing kk events over a time period for any non-negative integer value of kk.

Let XX be the discrete random variable that represents the number of events observed over a given time period. Let lambdaλ be the expected value (average) of XX. If XX follows a Poisson distribution, then the probability of observing kk events over the time period is 


where ee is Euler’s number.

In the World Cup, an average of 2.5 goals are scored each game. Modeling this situation with a Poisson distribution, what is the probability that kk goals are scored in a game?

There is no upper limit on the value of kk for this formula, though the probability rapidly approaches 0 as kk increases. _square 

Properties of the Poisson Distribution

The expected value of a Poisson distribution should come as no surprise, as each Poisson distribution is deExpected Value of Poisson Random Variable:

Given a discrete random variable XX that follows a Poisson distribution with parameter lambda,λ, the expected value of this variable is


By the definition of expected value, 

where x in text{Im}(X)x∈Im(X) simply means that xx is one of the possible values of the random variable XX. Applying this to the Poisson distribution,

The proof involves the routine (but computationally intensive) calculation that E[X^2]=lambda^2+lambdaE[X2]=λ2+λ. Then using the formula for variance 

text{Var}[X] = E[X^2]-E[X]^2,Var[X]=E[X2]−E[X]2,

we have text{Var}[X]=lambda^2+lambda-lambda^2=lambdaVar[X]=λ2+λλ2=λ.

The probability generating function for the Poisson distribution is  e^{lambda z}e^{-lambda}.eλzeλ.

Sum of Independent Poisson Random Variables:

Let XX and YY be Poisson random variables with parameters lambda_1λ1 and lambda_2λ2, respectively. If XX and YY are independent, then X+YX+Y is a Poisson random variable with parameter lambda_1+lambda_2.λ1+λ2. Its distribution can be described with the formula

P(X+Y=k)=frac{(lambda_1+lambda_2)^k e^{-(lambda_1+lambda_2)}}{k!}.P(X+Y=k)=k!(λ1+λ2)ke−(λ1+λ2)

The classical example of the Poisson distribution is the number of Prussian soldiers accidentally killed by horse-kick, due to being the first example of the Poisson distribution’s application to a real-world large data set. Ten army corps were observed over 20 years, for a total of 200 observations, and 122 soldiers were killed by horse-kick over that time period. The question is how many deaths would be expected over a period of a year, which turns out to be excellently modeled by the Poisson distribution ((with lambda=0.61):λ=0.61):ctical Applications

# of deaths Predicted % Expected # of occurrences Actual # of occurrences
0 54.34 108.67 109
1 33.15 66.29 65
2 10.11 20.22 22
3 2.05 4.11 3
4 0.32 0.63 1
5 0.04 0.08 0
6 0.01 0.01 0

The understanding of this information is significant: since the Poisson dispersion estimates the recurrence of occasions under the presumption of factual arbitrariness, the understanding of the normal appropriation with the real information proposes that the real information was to be sure because of haphazardness. On the off chance that the genuine information brought about a lot a bigger number of passings than anticipated, a substitute clarification ought to be looked (for example lacking preparing, a shrewd and unobtrusive foe plot, and so on.).

The Poisson distribution is also useful in determining the probability that a certain number of events occur over a given time period. For example, if an office averages 12 calls per hour, they can calculate that the probability of receiving at least 20 calls in an hour is

which means they can generally feel comfortable keeping only enough staff on hand to handle 20 calls. Of course, the choice of threshold depends on context; an emergency room, for instance, may still wish to have extra staff on hand.


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