Learn in your language
Skip to content
A Level · Cambridge (CIE) · 9709

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.

Statistics 2 · Poisson distribution · 6.1
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.
One illustrative one-minute observation shows irregular event locations. A stipulated homogeneous rate of three per minute gives X ~ Po(3), supported on 0,1,2,…, not a guaranteed count of three. The model assumes constant rate, independent disjoint-window counts and negligible multiple events in sufficiently small windows. The drawing is not evidence about a real process.Count events over one fixed exposureStipulated rate: 3 per minute01 minuteOne illustrative realizationX ~ Po(3) · support 0, 1, 2, …Mean 3 ≠ guaranteed countConstant rate · disjoint independenceCheck assumptions in real applications
One illustrative one-minute observation shows irregular event locations. A stipulated homogeneous rate of three per minute gives X ~ Po(3), supported on 0,1,2,…, not a guaranteed count of three. The model assumes constant rate, independent disjoint-window counts and negligible multiple events in sufficiently small windows. The drawing is not evidence about a real process.

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?

Put the idea to work

Worked example

A stipulated homogeneous process has three arrivals per minute. Define X for one minute and state its support.

Show the worked solution
  1. X counts arrivals during one minute.
  2. λ = 3 × 1 = 3.
  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.

Try a topic test

Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

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.

1 · Guided

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
  1. Let X count events in one minute.
  2. λ = 3 per minute × 1 minute = 3.
  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.
2 · Independent

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
  1. There are twenty trials with common probability 0.1.
  2. Thus X ~ B(20,0.1).
  3. 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.
3 · Transfer

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
  1. Bursts call independence into question.
  2. Changing rate conflicts with the constant-rate assumption.
  3. 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.
Recall without your notes

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.

After trying it yourself, choose your next review. This is your self-assessment.

Your review choice appears on Today. Sign in to sync it across devices.

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.

Keep learning Mathematics

Course scope and assessment

312 sequenced lessons provide 936 guided, independent and application problems covering the 160 objectives in sections 1.1–1.8, 2.1–2.6, 3.1–3.9, 4.1–4.5, 5.1–5.5 and 6.1–6.5. The map lists all 38 sections across six alternative components; all eight Pure Mathematics 1 sections and all six Pure Mathematics 2 sections, from Paper 2 Algebra through Numerical solution of equations, currently have full teaching sequences. Paper 3 Algebra, section 3.1, links the six shared foundation lessons plus six lessons on partial fractions and rational binomial expansions; Paper 3 logarithms, trigonometry and numerical equations use the shared lessons, with the Paper 1 graph foundation linked into trigonometry. Paper 3 differentiation combines the shared methods with its inverse-tangent derivative lesson. Paper 3 Integration links four shared standard and identity lessons plus seven advanced lessons, covering all six section 3.5 requirements. Eight Vectors lessons cover all six section 3.7 requirements, with 2D and 3D line and solid applications. Nine Differential equations lessons cover all four section 3.8 requirements, including Paper 3 integration techniques, initial conditions, equilibrium solutions and conditional model interpretations. Eleven Complex numbers lessons cover all eight section 3.9 requirements, with seven Argand diagrams and explicit argument and boundary checks. All nine Pure Mathematics 3 sections now have teaching linked to every mapped requirement. Nine Forces and equilibrium lessons cover all seven section 4.1 requirements, with six force diagrams, signed incline-friction intervals, reaction-dependent pull and push limits and separate-body third-law checks. Nine Kinematics lessons cover the four official section 4.2 requirements, with the graph requirement mapped to three relationships. Six motion diagrams support directed quantities, graph slopes and signed areas, Paper 1 time-calculus methods, reversal distances, constant-acceleration stages and shared-clock particle meetings. Six Momentum lessons cover both section 4.3 requirements, including signed system totals, conservation assumptions, outgoing velocities, coalescence, unknown masses and successive impacts. Four diagrams show system momentum through the stipulated events, and supplied kinetic-energy checks distinguish conservation of momentum from changes in kinetic energy. Twelve Newton’s laws lessons cover all four section 4.4 requirements, with vertical support and release stages, rough horizontal and incline motion, inferred parameters, hanging and surface pulley systems, rigid tow-bar thrust and slack-string transitions. Seven force diagrams support the separate-particle balances. Nine Energy, work and power lessons cover all five section 4.5 requirements, with angle-dependent work, kinetic and reference potential energy, signed net-work balances, smooth curved slides, rough and driven slopes, ideal connected-system energy and instantaneous power on hills. Five diagrams distinguish the energy inputs, losses and valid positive-speed force model. All five Mechanics sections now have teaching and staged practice linked to their mapped requirements. Thirteen Representation of data lessons cover all five section 5.1 requirements, including choice and limitations of displays, ordered and back-to-back stems, raw summaries and box plots, unequal-width density histograms, cumulative-frequency interpolation, descriptive moment calculations, grouped estimates, coding, combining two datasets and correcting totals. Six accessible diagrams retain repeated observations and distinguish numerical spread, frequency area and estimated readings. Ten Permutations and combinations lessons cover both official section 5.2 requirements, with selection and ordered roles, repeated objects, adjacency blocks, non-adjacency gaps, leading-zero cases, labelled multi-row seating and mixed restrictions. Six accessible diagrams show outcome trees, ordered versus unordered choices, repeated copies, blocks, gaps and distinct row seats. Eleven Probability lessons cover all four section 5.3 requirements, with equiprobable elementary outcomes, selection-based probabilities, complements and disjoint regions, independence tests, restricted sample spaces, two-way tables, replacement and removal trees, reversed conditions and unknown parameters. Seven accessible diagrams show dice outcomes, random subsets, event regions, a conditioned grid, a count table and two probability trees. Twelve Discrete random variables lessons cover all three official section 5.4 requirements, with complete probability tables, moment and parameter checks, binomial assumptions, exact probabilities and moments, geometric first-success probabilities, tails, expectation and integer reliability thresholds. Six accessible diagrams distinguish outcome grouping, weighted moments, fixed trials, discrete probability bars, waiting conventions and integer tail deadlines. Twelve Normal distribution lessons cover all three official section 5.5 requirements, with continuous-area interpretation, MF19 cumulative tables, full standardisation, tail and interval regions, conditional probabilities, percentiles, one- and two-parameter inference, binomial suitability, continuity corrections and approximate integer thresholds. Seven accessible diagrams show model areas, table reading, aligned scales, intervals, quantiles, matched binomial and normal shapes and whole integer cells. All five Statistics 1 sections now have teaching and staged practice linked to their mapped requirements. Fourteen Poisson distribution lessons cover all five official section 6.1 requirements, with homogeneous event-count assumptions, exposure scaling, exact masses and tails, conditional count events, moments, parameter inference, probability duration targets, disjoint and overlapping windows, rare-count binomial approximations, matched normal approximations and continuity corrections. Nine accessible diagrams show event exposures, unit conversions, discrete bars, complete tail complements, population moments, shared windows, approximate count models and integer cells. Exact checks distinguish approximate and exact close-target decisions. Fourteen Linear combinations lessons cover the six listed results in official section 6.2, with affine moments, signed scales and constant cases, expectation without independence, independent squared variance contributions, fixed and random fees, independent copies versus reused values, affine normal events, independent normal sums and differences, weighted and conditional areas, coefficient inference, exact independent Poisson sums and overlapping exposure limits. Nine accessible diagrams distinguish scale changes, variance contributions, component costs, copy mechanisms, aligned normal scales, summed areas, Poisson count pairs and unsupported distribution claims. Fourteen Continuous random variables lessons cover both official section 6.3 requirements, with density validity and units, support-aware intervals, complements and conditional regions, first and second moments, percentiles by direct density areas, symmetry checks, parameter inference, infinite-support power and exponential integrals, divergent-moment limits and combined applications. Nine accessible diagrams distinguish density height from probability, normalisation, negative-density failures, central and conditional areas, mean versus median, percentile locations and partial infinite tails. Explicit cumulative distribution function knowledge is not required. Eighteen Sampling and estimation lessons cover all eight official section 6.4 requirements, with population and frame definitions, random-number selection, sampling-quality limits, independent sample-mean moments, normal means, CLT conditions, SE scaling, raw, summarised and coded unbiased estimates, known-variance and large-sample mean intervals, repeated-sampling interpretation, precision planning, proportion intervals, sparse-outcome limits and contextual applications. Ten accessible diagrams distinguish selection frames, number rejection, repeated sample means, population versus mean density, CLT models, variance divisors, interval targets, illustrative coverage, integer size checks and proportion margins. Eighteen Hypothesis tests lessons cover all five official section 6.5 requirements, with pre-specified alternatives, inclusive count tails, explicit equal-tail conventions, integer critical regions and actual significance, exposure-aware Poisson tests, continuity-corrected count approximations, normal and large-sample mean tests, confidence-interval connections, error classifications, fixed-region binomial, Poisson and normal error probabilities and sample-size power comparisons. Nine accessible diagrams show the testing procedure, exact count regions, unequal Poisson tails, discrete significance jumps, corrected count boundaries, matching mean intervals and alternative error distributions. All shared lessons are counted once in the course total. All 38 sections now link teaching and practice. Eight earlier overview lessons remain available, but do not count as full section coverage. This is a content-availability measure, not full programme completion or exam readiness. Written practice is self-checked, not officially marked. Check the 2028–2030 syllabus separately if sitting an examination from 2028; this teaching map uses 2026–2027 requirements.

StudyForA · Learn with understanding

Original teaching content and links to official sources. Check your exam year and qualification before using a resource.