Poisson Distribution

343 questions · 33 question types identified

State conditions only

Questions that only ask to state, list, or write down the conditions/assumptions needed for a Poisson distribution to be valid, without requiring explanation or contextual application.

36
10.5% of questions
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3 The number of goals scored per match by Everly Rovers is represented by the random variable \(X\) which has mean 1.8.
  1. State two conditions for \(X\) to be modelled by a Poisson distribution. Assume now that \(X \sim \operatorname { Po } ( 1.8 )\).
  2. Find \(\mathrm { P } ( 2 < X < 6 )\).
  3. The manager promises the team a bonus if they score at least 1 goal in each of the next 10 matches. Find the probability that they win the bonus.
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Explain or apply conditions in context

Questions that require explaining what specific conditions mean in the given context, commenting on validity of assumptions, or discussing whether conditions are likely to hold in a particular scenario.

27
7.9% of questions
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3. An estate agent sells properties at a mean rate of 7 per week.
  1. Suggest a suitable model to represent the number of properties sold in a randomly chosen week. Give two reasons to support your model.
  2. Find the probability that in any randomly chosen week the estate agent sells exactly 5 properties.
  3. Using a suitable approximation find the probability that during a 24 week period the estate agent sells more than 181 properties.
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Single time period probability

Questions asking for P(X = k), P(X ≤ k), P(X ≥ k), or P(a ≤ X < b) for a Poisson variable over a single given time period with no scaling required.

26
7.6% of questions
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3 Th m brg calls receid d at a small call cen re \(\mathbf { h }\) s a Pósso d strib in with mean
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Two independent Poisson sums

Questions involving the sum of exactly two independent Poisson random variables, typically asking for probabilities of the combined total.

23
6.7% of questions
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1 Failures of two computers occur at random and independently. On average the first computer fails 1.2 times per year and the second computer fails 2.3 times per year. Find the probability that the total number of failures by the two computers in a 6-month period is more than 1 and less than 4 .
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Poisson hypothesis test

A question is this type if and only if it requires conducting a formal hypothesis test about the Poisson parameter λ, typically testing whether the rate has changed.

20
5.8% of questions
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  1. Bacteria are randomly distributed in a river at a rate of 5 per litre of water. A new factory opens and a scientist claims it is polluting the river with bacteria. He takes a sample of 0.5 litres of water from the river near the factory and finds that it contains 7 bacteria. Stating your hypotheses clearly test, at the \(5 \%\) level of significance, whether there is evidence that the level of pollution has increased.
\section*{Q uestion 1 continued}
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Single period normal approximation

Questions that use normal approximation to find probability for a Poisson distribution over a single time period (e.g., one year, one day, one hour), without comparing multiple periods or using binomial structure.

18
5.2% of questions
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3 Particles are emitted randomly from a radioactive substance at a constant average rate of 3.6 per minute. Find the probability that
  1. more than 3 particles are emitted during a 20 -second period,
  2. more than 240 particles are emitted during a 1-hour period.
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Poisson parameter from given probability

A question is this type if and only if it requires finding the parameter λ given a probability condition such as P(X = k) equals a specific value or P(X = a) = P(X = b).

15
4.4% of questions
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6 The discrete random variable \(R\) has the distribution \(\operatorname { Po } ( \lambda )\).
Use an algebraic method to find the range of values of \(\lambda\) for which the single most likely value of \(R\) is 7. [7]
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Mean-variance comparison for Poisson validation

Questions that provide sample mean and variance (or standard deviation) and ask whether these support a Poisson model by comparing their closeness, since for Poisson distributions mean equals variance.

12
3.5% of questions
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6 It is known that 1 in 5000 people in Atalia have a certain condition. A random sample of 12500 people from Atalia is chosen for a medical trial. The number having the condition is denoted by \(X\).
  1. Use an appropriate approximating distribution to find \(\mathrm { P } ( X \leqslant 3 )\).
  2. Find the values of \(\mathrm { E } ( X )\) and \(\operatorname { Var } ( X )\), and explain how your answers suggest that the approximating distribution used in (a) is likely to be appropriate.
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Multiple periods with binomial structure

Questions that first use normal approximation for a Poisson distribution, then apply this to multiple independent periods using binomial probability (e.g., probability that x out of n weeks satisfy a condition).

11
3.2% of questions
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  1. The probability of an electrical component being defective is 0.075 The component is supplied in boxes of 120
    1. Using a suitable approximation, estimate the probability that there are more than 3 defective components in a box.
    A retailer buys 2 boxes of components.
  2. Estimate the probability that there are at least 4 defective components in each box.
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Expectation and variance of Poisson-related expressions

A question is this type if and only if it asks to find E(Y) and/or Var(Y) where Y is a linear function of Poisson random variables or involves sums/differences of Poisson variables.

10
2.9% of questions
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1 The random variable \(X\) has the distribution \(\operatorname { Po } ( 1.3 )\). The random variable \(Y\) is defined by \(Y = 2 X\).
  1. Find the mean and variance of \(Y\).
  2. Give a reason why the variable \(Y\) does not have a Poisson distribution.
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Goodness-of-fit test for Poisson

A question is this type if and only if it involves using a chi-squared goodness-of-fit test to determine whether observed frequency data fits a Poisson distribution.

10
2.9% of questions
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4 The table shows the results of a random sample drawn from a population which is thought to have the distribution \(\mathrm { U } ( 20 )\). \end{table}
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Poisson with binomial combination

A question is this type if and only if it involves both Poisson and binomial distributions in the same problem, typically where Poisson events are followed by binomial trials.

10
2.9% of questions
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5. Defects occur at random in planks of wood with a constant rate of 0.5 per 10 cm length. Jim buys a plank of length 100 cm .
  1. Find the probability that Jim's plank contains at most 3 defects. Shivani buys 6 planks each of length 100 cm .
  2. Find the probability that fewer than 2 of Shivani's planks contain at most 3 defects.
  3. Using a suitable approximation, estimate the probability that the total number of defects on Shivani's 6 planks is less than 18.
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Multiple independent time periods

Questions that involve calculating probabilities across multiple independent time periods (e.g., probability that events occur in each of several separate periods, or only one of several periods).

10
2.9% of questions
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2. On a stretch of motorway accidents occur at a rate of 0.9 per month.
  1. Show that the probability of no accidents in the next month is 0.407 , to 3 significant figures. Find the probability of
  2. exactly 2 accidents in the next 6 month period,
  3. no accidents in exactly 2 of the next 4 months.
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Scaled time period sums

Questions where independent Poisson variables are first scaled to different time periods before summing, requiring rate adjustment before applying the sum property.

9
2.6% of questions
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1 Accidents at two factories occur randomly and independently. On average, the numbers of accidents per month are 3.1 at factory \(A\) and 1.7 at factory \(B\). Find the probability that the total number of accidents in the two factories during a \(2 -\) month period is more than 3 .
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Scaled time period probability

Questions requiring scaling of the Poisson mean to a different time period (e.g., from per week to per 2 weeks, or per minute to per 90 minutes) before calculating probabilities.

9
2.6% of questions
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6 Calls arrive at a helpdesk randomly and at a constant average rate of 1.4 calls per hour. Calculate the probability that there will be
  1. more than 3 calls in \(2 \frac { 1 } { 2 }\) hours,
  2. fewer than 1000 calls in four weeks ( 672 hours).
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Joint probability of separate processes

Questions asking for the probability that specific events occur in two or more independent Poisson processes separately (e.g., at least 2 men AND at least 3 women), requiring multiplication of individual Poisson probabilities.

9
2.6% of questions
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7 Men arrive at a clinic independently and at random, at a constant mean rate of 0.2 per minute. Women arrive at the same clinic independently and at random, at a constant mean rate of 0.3 per minute.
  1. Find the probability that at least 2 men and at least 3 women arrive at the clinic during a 5 -minute period.
  2. Find the probability that fewer than 36 people arrive at the clinic during a 1-hour period.
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Sum of independent Poisson processes

Questions asking about the total count across multiple independent Poisson processes, using the result that the sum of independent Poisson variables is also Poisson with mean equal to the sum of individual means.

9
2.6% of questions
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6 People arrive at a checkout in a store at random, and at a constant mean rate of 0.7 per minute. Find the probability that
  1. exactly 3 people arrive at the checkout during a 5 -minute period,
  2. at least 30 people arrive at the checkout during a 1-hour period. People arrive independently at another checkout in the store at random, and at a constant mean rate of 0.5 per minute.
  3. Find the probability that a total of more than 3 people arrive at this pair of checkouts during a 2-minute period.
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Conditional probability with Poisson

A question is this type if and only if it asks for P(A|B) where both events involve Poisson random variables, requiring use of conditional probability formula.

8
2.3% of questions
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4 The random variable \(A\) has the distribution \(\operatorname { Po } ( 1.5 ) . A _ { 1 }\) and \(A _ { 2 }\) are independent values of \(A\).
  1. Find \(\mathrm { P } \left( A _ { 1 } + A _ { 2 } < 2 \right)\).
  2. Given that \(A _ { 1 } + A _ { 2 } < 2\), find \(\mathrm { P } \left( A _ { 1 } = 1 \right)\).
  3. Give a reason why \(A _ { 1 } - A _ { 2 }\) cannot have a Poisson distribution.
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Proving Poisson properties from first principles

A question is this type if and only if it requires algebraic proof of Poisson distribution properties such as E(X), Var(X), or E(X(X-1)) using the probability mass function.

7
2.0% of questions
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1 The random variable \(X\) has a Poisson distribution with mean 16 Find the standard deviation of \(X\) Circle your answer.
4
8
16
256
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Three or more independent Poisson sums

Questions involving the sum of three or more independent Poisson random variables, requiring combination of multiple sources or types.

7
2.0% of questions
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1 The numbers of \(\alpha\)-particles emitted per minute from two types of source, \(A\) and \(B\), have the distributions \(\operatorname { Po } ( 1.5 )\) and \(\operatorname { Po } ( 2 )\) respectively. The total number of \(\alpha\)-particles emitted over a period of 2 minutes from three sources of type \(A\) and two sources of type \(B\), all of which are independent, is denoted by \(X\). Calculate \(\mathrm { P } ( X = 27 )\).
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Finding minimum n for P(X=0) threshold

Questions requiring the smallest n such that the probability of zero occurrences falls below a given threshold (e.g., P(X=0) < 0.05 or P(X=0) < 0.01).

7
2.0% of questions
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4 On average, 1 in 2500 people have a particular gene.
  1. Use a suitable approximation to find the probability that, in a random sample of 10000 people, more than 3 people have this gene.
  2. The probability that, in a random sample of \(n\) people, none of them has the gene is less than 0.01 . Find the smallest possible value of \(n\).
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Frequency distribution and Poisson fit

Questions that provide a frequency table of observed counts and ask whether the data support a Poisson model, typically requiring calculation of sample mean and variance from the frequency distribution.

7
2.0% of questions
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  1. Specify a suitable model for the distribution of \(X\).
  2. Find the mean and the standard deviation of \(X\). \item A secretarial agency carefully assesses the work of a new recruit, with the following results after 150 pages: \end{enumerate}
    No of errors0123456
    No of pages163841291772
  3. Find the mean and variance of the number of errors per page.
  4. Explain how these results support the idea that the number of errors per page follows a Poisson distribution.
  5. After two weeks at the agency, the secretary types a fresh piece of work, six pages long, which is found to contain 15 errors.
    The director suspects that the secretary was trying especially hard during the early period and that she is now less conscientious. Using a Poisson distribution with the mean found in part (a), test this hypothesis at the \(5 \%\) significance level.
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Standard Poisson approximation to binomial

Questions that ask to use Poisson approximation to approximate a binomial distribution B(n,p) where n is large and p is small, with straightforward probability calculations.

7
2.0% of questions
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1 On average, 1 in 50000 people have a certain gene.
Use a suitable approximating distribution to find the probability that more than 2 people in a random sample of 150000 have the gene.
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Consecutive non-overlapping periods

Questions specifically about consecutive or successive time periods where the temporal ordering or sequence matters (e.g., first half vs second half, successive minutes).

6
1.7% of questions
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2. Accidents on a particular stretch of motorway occur at an average rate of 1.5 per week.
  1. Write down a suitable model to represent the number of accidents per week on this stretch of motorway. Find the probability that
  2. there will be 2 accidents in the same week,
  3. there is at least one accident per week for 3 consecutive weeks,
  4. there are more than 4 accidents in a 2 week period.
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Finding minimum stock level for demand

Questions requiring the minimum number of items to stock so that the probability of meeting demand exceeds a threshold (practical inventory problems).

6
1.7% of questions
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2. The number of copies of The Statistician that a newsagent sells each week is modelled by a Poisson distribution. On average, he sells 1.5 copies per week.
  1. Find the probability that he sells no copies in a particular week.
  2. If he stocks 5 copies each week, find the probability he will not have enough copies to meet that week's demand.
  3. Find the minimum number of copies that he should stock in order to have at least a \(95 \%\) probability of being able to satisfy the week's demand.
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Finding maximum n for P(X=0) threshold

Questions requiring the largest n such that the probability of zero occurrences exceeds a given threshold (e.g., P(X=0) > 0.9 or P(X=0) ≥ 0.95).

5
1.5% of questions
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4 The number of accidents on a certain road has a Poisson distribution with mean 0.4 per 50-day period.
  1. Find the probability that there will be fewer than 3 accidents during a year (365 days).
  2. The probability that there will be no accidents during a period of \(n\) days is greater than 0.95 . Find the largest possible value of \(n\).
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Finding minimum n for P(X≥k) threshold

Questions requiring the smallest n such that the probability of at least k occurrences satisfies a condition (e.g., P(X≥1) = 0.9, or P(X≥n) < 0.05).

5
1.5% of questions
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6 People arrive randomly and independently at the elevator in a block of flats at an average rate of 4 people every 5 minutes.
  1. Find the probability that exactly two people arrive in a 1-minute period.
  2. Find the probability that nobody arrives in a 15 -second period.
  3. The probability that at least one person arrives in the next \(t\) minutes is 0.9 . Find the value of \(t\).
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Poisson approximation justification or comparison

Questions that explicitly ask to justify why Poisson approximation is suitable, explain why both binomial and Poisson could be used, or compare distributions.

5
1.5% of questions
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4 A radioactive source contains 1000000 nuclei of a particular radioisotope. On average 1 in 200000 of these nuclei will decay in a period of 1 second. The random variable \(X\) represents the number of nuclei which decay in a period of 1 second. You should assume that nuclei decay randomly and independently of each other.
  1. Explain why you could use either a binomial distribution or a Poisson distribution to model the distribution of \(X\). Use a Poisson distribution to answer parts (b) and (c).
  2. Calculate each of the following probabilities.
    • \(\mathrm { P } ( X = 6 )\)
    • \(\mathrm { P } ( X > 6 )\)
    • Determine an estimate of the probability that at least 60 nuclei decay in a period of 10 seconds.
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Poisson with geometric or waiting time

A question is this type if and only if it involves finding probabilities related to waiting times or the number of trials until a Poisson event occurs.

3
0.9% of questions
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  1. A chocolate manufacturer places special tokens in \(2 \%\) of the bars it produces so that each bar contains at most one token. Anyone who collects 3 of these tokens can claim a prize.
Andreia buys a box of 40 bars of the chocolate.
  1. Find the probability that Andreia can claim a prize. Barney intends to buy bars of the chocolate, one at a time, until he can claim a prize.
  2. Find the probability that Barney can claim a prize when he buys his 40th bar of chocolate.
  3. Find the expected number of bars that Barney must buy to claim a prize.
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Sample mean distribution of Poisson

A question is this type if and only if it asks about the distribution of the sample mean from multiple independent Poisson observations, typically using CLT for large samples.

2
0.6% of questions
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5 A random variable \(X\) has the distribution \(\operatorname { Po } ( 3.2 )\).
  1. A random value of \(X\) is found.
    (a) Find \(\mathrm { P } ( X \geqslant 3 )\).
    (b) Find the probability that \(X = 3\) given that \(X \geqslant 3\).
  2. Random samples of 120 values of \(X\) are taken.
    (a) Describe fully the distribution of the sample mean.
    (b) Find the probability that the mean of a random sample of size 120 is less than 3.3.
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Single scaled time period

Questions that require scaling the Poisson parameter to a single different time period and calculating probabilities for that period only.

2
0.6% of questions
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1
  1. The number of inhabitants of a village who are selected for jury service in the course of a 10-year period is a random variable with the distribution \(\operatorname { Po } ( 4.2 )\).
    (a) Find the probability that in the course of a 10-year period, at least 7 inhabitants are selected for jury service.
    (b) Find the probability that in 1 year, exactly 2 inhabitants are selected for jury service.
  2. Explain why the number of inhabitants of the village who contract influenza in 1 year can probably not be well modelled by a Poisson distribution.
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Conditional or competitive scenarios

Questions involving conditional probabilities or competitive outcomes between processes (e.g., one team scoring more than another, overflow between guesthouses), requiring consideration of relative outcomes rather than just joint or total counts.

2
0.6% of questions
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4 Goals scored by Femchester United occur at random with a constant average of 1.2 goals per match. Goals scored against Femchester United occur independently and at random with a constant average of 0.9 goals per match.
  1. Find the probability that in a randomly chosen match involving Femchester,
    (a) a total of 3 goals are scored,
    (b) a total of 3 goals are scored and Femchester wins. The manager promises the Femchester players a bonus if they score at least 35 goals in the next 25 matches.
  2. Find the probability that the players receive the bonus.
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Poisson approximation with parameter finding

Questions that require finding an unknown parameter (typically n) given a probability condition, using the Poisson approximation to binomial.

0
0.0% of questions