A Level · Cambridge (CIE) · 9709

Add independent Poisson counts with exact distribution closure

Use the exact Poisson-sum result and distinguish probabilities of a total from probabilities of one particular contributing pair.

Statistics 2 · Linear combinations · 6.2

Exam scope and assessment notes

Probability & Statistics 2 (Paper 6, A Level) · Mathematics 9709, 2026–2027, section 6.2. Paper 6 builds on Paper 5 and assumes Paper 3 calculus. Use E(aX + b) = aE(X) + b and Var(aX + b) = a²Var(X). Expectation is linear without independence, when the relevant moments exist. For independent X and Y, Var(aX + bY) = a²Var(X) + b²Var(Y); negative coefficients still give positive squared contributions. A constant changes the mean but not variance. SD scales by |a|, not a². Affine transforms of normal variables and linear combinations of independent normal variables are normal when the resulting variance is positive; zero variance is a constant, so handle it directly rather than standardising by zero. Independent Poisson counts have an exact Poisson sum with the parameters added; a scaled count or difference is not generally Poisson. Equal marginal distributions do not establish independence. Proofs are not required. Normal probabilities here use quoted cumulative values when supplied and otherwise unrounded standardisation with numerical areas. Exact distribution results are conditional on the stipulated models; real applications need evidence. Continuous density functions, sampling, estimation and hypothesis testing belong to later sections. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending.

Before you begin

Use exact Poisson masses.
Multiply independent events and condition on a positive event.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

Which parameter matches the independent sum of Po(2) and Po(3)?

What you will learn

State Po(λ₁ + λ₂) for independent Poisson counts.
Calculate total-count and conditional component probabilities.

Independent X ~ Po(2), Y ~ Po(3) sum exactly to Po(5). Total-two events have pairs (0,2), (1,1), (2,0), whose independent probability masses are respectively 4.5e⁻⁵, 6e⁻⁵, 2e⁻⁵. They sum to 12.5e⁻⁵ ≈ 0.0842. One contributing pair is not the whole total event.

Add the parameters for independent unweighted counts

Let independent X ~ Po(2), Y ~ Po(3). Their sum S follows exactly Po(5). Mean and variance are each five, matching the separate independent moment calculations. This is distribution closure, not the rare-binomial approximation from section 6.1.

P(S = 2) = e⁻⁵5²/2! = 12.5e⁻⁵ ≈ 0.0842. The event includes component pairs (0,2),(1,1),(2,0). Their independent masses have coefficients 4.5,6,2 times e⁻⁵, adding to 12.5e⁻⁵. Multiplying P(X = 2)P(Y = 2) would instead describe total four.

Keep conditions and parameter limits

P(S ≥ 1) = 1 − e⁻⁵ ≈ 0.9933. Given S = 1, P(X = 1 | S = 1) has numerator P(X = 1,Y = 0) = 2e⁻⁵ and denominator P(S = 1) = 5e⁻⁵, giving 2/5.

The counts must be independent and their parameters finite and non-negative. A zero-parameter component is a constant zero, so adding it leaves the other count unchanged. If the sum parameter is zero, conditioning on a positive sum is undefined. Weighted sums and differences do not have this general Poisson closure.

Pause and explain

Which pairs contribute to X + Y = 2?

Put the idea to work

Worked example

For independent Po(2), Po(3), find the sum model and its probability of two.

Show the worked solution

Add parameters to get Po(5).
The count-two mass is 25e⁻⁵/2.
Evaluate ≈ 0.0842.

Answer Po(5); P(S = 2) ≈ 0.0842.

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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

Independent X ~ Po(2), Y ~ Po(3). State S = X + Y’s model and P(S = 2).

Give me a hint

Use exact sum closure before the mass formula.

Compare my reasoning

S ~ Po(5).
P(S = 2) = 12.5e⁻⁵.
The value is approximately 0.0842.

Po(5); probability ≈ 0.0842.

Look for these in your work

State independence.
Label exact model closure.

2 · Independent

For the same sum, find the probability of at least one event.

Give me a hint

Use its zero-count complement.

Compare my reasoning

S has parameter five.
P(S = 0) = e⁻⁵.
P(S ≥ 1) = 1 − e⁻⁵ ≈ 0.9933.

1 − e⁻⁵ ≈ 0.9933.

Look for these in your work

Use the sum parameter.
Preserve the whole upper event.

3 · Transfer

For those independent counts, find P(X = 1 | S = 1).

Give me a hint

The numerator requires X = 1 and Y = 0.

Compare my reasoning

The joint numerator is 2e⁻⁵.
The sum-one mass is 5e⁻⁵ > 0.
The ratio is 2/5.

2/5.

Look for these in your work

Intersect the condition correctly.
Divide by the sum-event probability.

Common mistakes

Independence is essential to the stated sum theorem.
A total event includes all contributing pairs.
Poisson-sum closure does not cover every weighted difference.

Recall without your notes

What can you explain now?

Does S = 2 require both components equal two?

Compare with the explanation

No; that would total four.

The total-two pairs are (0,2),(1,1),(2,0).

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Make it stick

Check independent Poisson inputs, add their parameters, and distinguish the total event from a particular contributing pair.

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Keep learning Mathematics

Check the official syllabus 2026–2027

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.