Exponential Distribution: Formula, Examples, Questions

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Jasmine Grover

Education Journalist | Study Abroad Lead

The exponential distribution is commonly used to calculate the time before a specific event occurs. For example, the amount of time (from now) until an earthquake happens has an exponential distribution. The number of large values is decreasing, while the number of tiny values is increasing. The amount of money spent by clients in a single trip to the grocery, for example, follows an exponential distribution. The number of people who spend modest quantities of money is increasing, while the number of people who spend huge sums of money is decreasing. In the topic of reliability, the exponential distribution is commonly utilised. The term "reliability" refers to how long a product will last.

Key Takeaways: Exponential distribution, Exponential Distribution Formula, Exponential Distribution Graph, Mean of an Exponential Distribution, Variance of an Exponential Distribution, 


Exponential Distribution

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One of the most common continuous distributions is the exponential distribution. It's frequently used to calculate the amount of time that has passed between two events. Some of the examples of exponential distribution include the duration in minutes of long-distance business phone calls and the number of months a car battery lasts. It's also possible to show that the value of your change in your pocket or handbag follows an exponential distribution. 


Exponential Distribution Formula

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Exponential Distribution Formula

Exponential Distribution Formula


Need to Design Exponential Distribution

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Exponential Distribution is needed to estimate the length of time until the next occurrence (i.e., success, failure, arrival, etc.)

For instance, we'd like to forecast the following:

  • The amount of time it takes for a customer to finish browsing and make a purchase in your store (success). 
  • The time it will take for the hardware on AWS EC2 to fail (failure).
  • The amount of time you'll have to wait for the bus (arrival).

For example, your blog has 500 daily visitors. That is a percentage. The rate (λ) of the unit of time, which is the parameter of the Poisson distribution, is the number of customers arriving at the store in an hour, the number of earthquakes per year, the number of traffic accidents per week, the number of typos on a page, the number of hairs detected in Chipotle, and so on.

When modelling the elapsed time between events, however, we tend to speak in terms of time rather than rate, for example, the number of years a computer can power on without failing is 10 years (rather than 0.1 failure/year, which is a rate), a customer arrives every 10 minutes, major hurricanes strike every 7 years, and so on. When you see the phrase "mean" of an exponential distribution, you should know that it means 1/λ.


Exponential Distribution Graph

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The exponential distribution graph is a probability density function graph that depicts the distribution of distance or time between events. Lambda (λ) and x are the two terms used in the exponential distribution graph. Lambda denotes the number of events per unit of time, whereas x denotes the amount of time.

Exponential Distribution Graph

Exponential Distribution Graph


Mean of an Exponential Distribution

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The mean of an exponential distribution is equal to the standard deviation, as proven by the equation = μ = σ = 1/λ. Furthermore, the exponential distribution is the only "memoryless" continuous distribution, with P( X > a+b | X > a) = P(X > b).


Variance of an Exponential Distribution

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Let X be a continuous random variable with a parameter of and an exponential distribution. Then var(X)=β2 is the variance of X.

To calculate the variance of an exponential distribution, we must first determine the second moment of the distribution, which is given by:

E|X2| = \(\int \)0 x2λe -λx\(\frac{2}{\lambda^2}\)

Sum of Two Exponential Random Variables' Distribution

The sum of two independent stochastic variables' moment generating functions is the product of their respective moment generating functions. m(t)=1/(1-t/λ)^(-1) =λ/(λ-t) is the moment generating function of an exponential distribution.

fZz = \(\int \)-∞fx1(x1) fx2(z – x1) dx1

\(\int \)20 λ1-λ1x1λ2 -λ2(z – x1)dx1

= λ1λ2e -λ2z \(\int \)ze (λ2 – λ1) x1 dx1

={ \({ \frac{\lambda_1 \lambda_2}{\lambda_2 - \lambda_1}}\) (e -λ1z  – e -λ2z) if λ1 ≠ λ2

λ2ze -λx                   if λ1 = λ2 = λ


The Memoryless Property

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The conditional behaviour of exponential random variables is shown by the memoryless property. It's one of our most important findings, and we'll utilise it to figure out how to solve queueing systems. Assume that X is distributed exponentially with parameter λ. Assume we know that X > t. What's the chance that X is also greater than some s + t value? To put it another way, we'd like to know.

P(X > s + t | X > t) 


Things to Remember 

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  • The exponential distribution is defined as the probability distribution of time between occurrences in the Poisson point process in probability theory. 
  • The elapsed time can be considered a variable with random numbers in any occurrence where the answer to dependability questions is unknown. 
  • The variable will follow an exponential distribution as long as the event continues to occur at a constant pace.
  • The probability density function is represented by an exponential distribution graph in terms of distance or time difference between two events.
  • The exponential distribution is one of the most extensively used continuous distributions. 
  • It aids in determining the amount of time that has passed between events. It's employed in a variety of fields, including dependability theory, queuing theory, and physics.

Sample Questions 

Ques. What is an exponential distribution and how does it work? (3 marks)

Ans. The exponential distribution is a continuous distribution used to estimate the time it will take for an event to occur. For example, in physics, it is frequently used to calculate radioactive decay, in engineering, it is frequently used to calculate the time required to receive a defective part on an assembly line, and in finance, it is frequently used to calculate the likelihood of a portfolio of financial assets defaulting. It can also be used to estimate the likelihood of a certain number of defaults occurring within a certain time frame.

Ques. An exponential distribution with an average time of eight minutes can be used to model the number of times spouses spend shopping for anniversary cards. Make a graph of the distribution, write the distribution, and state the probability density function. (2 marks)

Ans. 

An exponential distribution with an average time of eight minutes can be used to model the number of times spouses spend shopping for anniversary cards. Make a graph of the distribution, write the distribution, and state the probability density function.

Ques. On average, a computer component lasts ten years. The duration of the computer part is dispersed exponentially. (5 marks)
a) What is the likelihood that a computer component would last longer than 7 years?
b) How long would five computer parts last on average if they were used one after the other?
c) How long do you think 80% of computer parts last?

Answer: a) Let x be the length of time (in years) that a computer component lasts.

a) Let x be the length of time (in years) that a computer component lasts.

b) One computer component lasts ten years on average. As a result, if five computer parts are used one after the other, they will endure on average (5)(10) = 50 years.

c) Make a graph. Let k represent the 80th percentile of the population.

c) Make a graph. Let k represent the 80th percentile of the population.

Ques. Assume the lifespan of a light bulb is exponential, with an average lifespan of eight years. Find the probability that a bulb will live a total of more than 19 years if it has already lasted 12 years. (3 marks)

Ans. The exponential distribution and the Poisson distribution have an interesting relationship. Assume that the amount of time that passes between two events follows an exponential distribution with a mean of units of time. Assume that these times are unrelated, that is, the time between events is unaffected by the time between preceding events. If these assumptions are correct, the number of occurrences per unit time will follow a Poisson distribution with a mean of λ = 1/μ. If X's Poisson distribution has a mean, then

\(P(X = K)= \frac{\lambda^k_e - \lambda}{k!}\)

In contrast, if the number of occurrences per unit time follows a Poisson distribution, the time between events will follow an exponential distribution. (k!=k*(k–1*)) (k–2)*(k–3)*…3*2*1)

Assume that X has a Poisson distribution with a mean. Enter 2nd to calculate P(X = k).

VARS(DISTR), C: poissonpdf(λ,k). To compute P(X≤k), enter 2nd, VARS(DISTR), D: poissoncdf(λ,k)

Ques. How long must we wait for a specific event to occur? (3 marks)

Ans. If we represent the situation using the Exponential Distribution, we can get a probabilistic solution to this question. Because we don't know how long we'll have to wait, we might think of it as a Random Variable. The Random Variable has an exponential distribution if the chance of an event occurring in a given interval is proportional to the interval length. An Exponential Random Variable's support (the set of values the Random Variable can take) is the set of all positive real numbers.

R X = [0, ∞]

Ques. What exactly does "X Exp(0.25)" mean? (3 marks)

Ans. The Poisson rate for X Exp(0.25) will therefore be 0.25. The event occurs 0.25 times on average during a unit of time whether you can take it as a second, minute, hour, week, month or year. If you are considering your unit of time is an hour, it will take 4 hours which is a reciprocal of 0.25 for the event to occur.

Ques. Poisson invented the Poisson Distribution for a reason. Comment on this statement. (5 marks)

Ans. Poisson created the Poisson Distribution to forecast the number of future events! To predict the likelihood of a certain number of events occurring in a specific time interval. This "event" can be described as a consumer purchasing something from you if you've ever sold something (the moment of truth, not just browsing). It might be the number of daily visits to your website, the number of clicks on your advertising for the next month, the number of phone calls you receive during your shift, or even how many people will die from a terrible disease next year, among other things.

Ques. Consider the following scenario to see what the Binomial Distribution's shortcomings are:
This is a year's worth of data. My blog has been read by 59k people. Only 888 individuals out of 59k applauded.

Ans.: Let's say you have a binomial random variable that is "BI-nary" — either 0 or 1.

In the case above, there were 17 persons who clapped each week. This translates to 17/7 = 2.4 claps per day and 17/(7*24) = 0.1 claps per hour.

If we use the binomial random variable to describe the success probability by the hour (0.1 people/hr), most of the hours will receive zero claps, but some will receive exactly one clap. However, it's likely that some hours will receive more than one clap (2, 3, 5 claps, etc.)

The issue with binomial is that it CAN NOT hold more than one event in a given time unit (in this case, 1 hr is the unit time). Only 0 or 1 event can occur in a unit of time.

Then, instead of dividing 1 hour into 60 minutes, how about making unit time smaller, like a minute? Then a single hour can contain a number of events. (However, in one minute, there will be exactly one or zero incidents.)

Ques. Write some applications of Exponential Distribution. (5 marks)

Ans. When describing the waiting time in a homogeneous Poisson process, the exponential distribution appears naturally. It has applications in queuing theory, physics, dependability theory, and hydrology, among others. The following are some examples of occurrences that can be described using exponential distribution:

  • The amount of time it takes for a radioactive particle to decay.
  • The interval between Geiger counter clicks.
  • The time until the company's debt holders is paid in full.
  • On a given road, the distance between roadkills.
  • On a DNA strand, the distance between mutations.
  • The time it takes a teller at a bank to service a customer.
  • The height of distinct molecules in a gas in a uniform gravitational field at a constant temperature and pressure.
  • The maximum monthly and annual daily rainfall and river discharge levels.

Ques. Describe the assumptions for the Poisson Model (5 marks)

Ans. The assumptions for the Poisson Model are:

  1. The average number of events per unit of time does not change.

Because the hourly rate is not constant, the number of people who visit your blog every hour may not follow a Poisson Distribution (higher rate during the daytime, lower rate during the nighttime). Because the seasonality effect is non-trivial in that area, using monthly rates for consumer/biological data would also be an approximation.

  1. Events are independent.

It's possible that your blog visitors' arrivals aren't always random. For example, a big number of visitors may arrive in a group because your blog was cited by a popular figure, or your blog was highlighted on Medium's front page, and so on. If one strong earthquake increases the likelihood of aftershocks, the number of earthquakes per year in a country may not follow a Poisson Distribution.

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CBSE CLASS XII Related Questions

  • 1.
    Find:

    If \[ (3\hat{i}-2\hat{j}+5\hat{k})\times(4\hat{i}+p\hat{j}+q\hat{k})=\vec{0} \] then find the values of \(p\) and \(q\).

      • \(p = -\frac{2}{3}, \, q = \frac{5}{3}\)
      • \(p = -\frac{8}{3}, \, q = \frac{20}{3}\)
      • \(p = \frac{20}{3}, \, q = -\frac{8}{3}\)
      • \(p = 0, \, q = 0\)

    • 2.
      Using integration, find the area of the region bounded by the curve \( y = x|x| \), the x-axis, and the vertical lines \( x = -2 \) and \( x = 2 \).


        • 3.
          Find the vector and cartesian equations of the line passing through the point of intersection of the lines \( \vec{r} = (\hat{i} + \hat{j} - \hat{k}) + \lambda(3\hat{i} - \hat{j}) \) and \( \vec{r} = (4\hat{i} - \hat{k}) + \mu(2\hat{i} + 3\hat{k}) \) and parallel to the line \( \frac{x - 1}{-2} = \frac{7 - y}{-3} = z \).


            • 4.
              Which of the following equations is NOT a Linear Differential Equation?

                • \((1 + x^2) \, dy + 2xy \, dx = \cot x \, dx\)
                • \(y + \frac{d}{dx}(xy) = x(\sin x + \log x)\)
                • \(x(1 + y^2) \, dx - y(1 + x^2) \, dy = 0\)
                • \(y \, dx - (x + 3y^2) \, dy = 0\)

              • 5.

                An NGO organises a charity event in which they decide to distribute woollen caps to protect children from winter. The caps to be distributed are in three separate boxes, Box I has 30 red caps, Box II has 20 red and 10 green caps, and Box III has 30 green caps. The probability that a Box i is selected and a cap picked out is i/6, where i = 1, 2, 3.  
                Based on the above information, answer the following questions :


                  • 6.
                    Find: \[ \int \frac{x^2}{(x^2-1)(x^2+4)}\,dx \]

                      CBSE CLASS XII Previous Year Papers

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