Define a Poisson count and its modelling assumptions
Define an event count over a fixed exposure and check whether the stated assumptions support a homogeneous Poisson model.
Exam scope and assessment notes
Probability & Statistics 2 (Paper 6, A Level) · Mathematics 9709, 2026–2027, section 6.1. Paper 6 builds on Paper 5 and assumes Paper 3 calculus. Po(λ) counts non-negative integers over a stated exposure, with finite λ ≥ 0. Mean and variance are each λ; proofs are not required. The homogeneous process examples stipulate constant rate, independent counts in disjoint exposures, and negligible multiple events in sufficiently small exposures; real applications need evidence. A binomial-to-Poisson approximation uses large n and small p, with n > 50 and np < 5 approximately as the syllabus guide. A Poisson-to-normal approximation uses λ > 15 approximately, matching N(λ,λ) and applying continuity correction. These guides are not universal error guarantees. Exact and approximate calculations are labelled separately; numerical normal probabilities use unrounded z and can differ slightly from rounded-table working. Independent Poisson sums and linear combinations, sample means and hypothesis testing belong to later Statistics 2 sections. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending.
Before you begin
- Recognise discrete counts and their support.
- Recall fixed-trial binomial assumptions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does 3 describe in a one-minute Po(3) count?
What you will learn
- Identify exposure, rate and the non-negative integer support.
- Distinguish model assumptions from real-population evidence.
Count events over a stated exposure
X ~ Po(3) can model arrivals in one minute for a stipulated process of rate three per minute. Possible counts are 0,1,2,… with no finite upper endpoint. Three is the expected count, not a guarantee of exactly three arrivals. Define the event and the observation window.
A homogeneous Poisson process assumes constant rate, independent counts in disjoint windows, and negligible probability of two or more events in a sufficiently short window. These are assumptions to justify in real applications. The sketch is one illustrative irregular realization, not a required spacing pattern.
Choose a model for the mechanism
Successes in twenty independent trials with common success probability 0.1 follow exactly B(20,0.1), supported on 0 through 20. A fixed number of trials differs from counting events over an exposure. A known average alone does not determine the distribution.
Dependent bursts or a changing rate undermine this stated homogeneous model. Investigate the mechanism and data; other changing-rate models may exist beyond this lesson. Equality of observed mean and variance alone does not prove a Poisson distribution.
Pause and explain
Twenty independent trials each have success probability 0.1. Which exact model follows?
Worked example
A stipulated homogeneous process has three arrivals per minute. Define X for one minute and state its support.
Show the worked solution
- X counts arrivals during one minute.
- λ = 3 × 1 = 3.
- Write X ~ Po(3), with support 0,1,2,… and expected count three.
Answer X ~ Po(3); support 0,1,2,… under the stated assumptions.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define a one-minute count for a stipulated homogeneous process of rate three per minute.
Give me a hint
Multiply rate by exposure and identify the possible integer counts.
Compare my reasoning
- Let X count events in one minute.
- λ = 3 per minute × 1 minute = 3.
- X ~ Po(3), taking values 0,1,2,… under the assumptions.
Po(3); support 0,1,2,….
Look for these in your work
- Define the count and exposure.
- State the conditional model assumptions.
Twenty independent trials each have success probability 0.1. State their success-count distribution and support.
Give me a hint
Use the fixed independent trial mechanism from Statistics 1.
Compare my reasoning
- There are twenty trials with common probability 0.1.
- Thus X ~ B(20,0.1).
- Its support is 0,…,20, unlike unbounded Poisson support.
B(20,0.1), support 0,…,20.
Look for these in your work
- Use the mechanism rather than only its mean.
- State the finite support.
Arrivals occur in dependent bursts and their rate changes through the morning. Is the homogeneous model established?
Give me a hint
Compare these observations with the proposed constant rate and independence.
Compare my reasoning
- Bursts call independence into question.
- Changing rate conflicts with the constant-rate assumption.
- Review a suitable model and evidence; do not claim every alternative process model is impossible.
The stated homogeneous model is not justified.
Look for these in your work
- Identify the assumptions in doubt.
- Limit the conclusion to the proposed model.
Common mistakes
- A mean alone does not specify a distribution.
- Fixed trials and fixed exposure are different mechanisms.
- Disjoint-count independence needs justification.
What can you explain now?
Must a Po(3) count equal three?
Compare with the explanation
No; three is its expectation.
Any non-negative integer is possible under this model.
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Make it stick
Define event and exposure, then justify rate and independence before choosing the count distribution.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.