Calculate Poisson approximations and compare exact binomial answers
Calculate rare-count estimates alongside exact fixed-trial probabilities while keeping their models and support distinct.
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
- Check approximation suitability.
- Use binomial and Poisson formulae.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is exact P(0) for B(200,0.01)?
What you will learn
- Calculate labelled Poisson approximations.
- Compare independently calculated exact binomial answers.
Use the approximate distribution
For B(200,0.01), Po(2) estimates P(X ≥ 1) as 1 − e⁻² ≈ 0.8647. The exact binomial answer is 1 − 0.99²⁰⁰ ≈ 0.8660. Close values are not equal; name the model used.
For B(150,0.02), use Po(3): P(2) ≈ 4.5e⁻³ ≈ 0.2240. The exact binomial mass is 150C2 × 0.02² × 0.98¹⁴⁸ ≈ 0.2248. Keep nCr in the binomial formula only, and use no half-unit correction.
Keep support and event labels
For B(200,0.99), approximate failures by Po(2). At least 199 successes means at most one failure, estimated as 3e⁻² ≈ 0.4060. Exact binomial probability is 0.99²⁰⁰ + 200 × 0.01 × 0.99¹⁹⁹ ≈ 0.4046.
Binomial support stops at n; Poisson support continues indefinitely. The diagram compares counts zero through seven only. Suitability does not guarantee a tight target decision agrees with exact calculation; verify exact values when the approximate probability is close to the target.
Pause and explain
Does a Poisson approximation to binomial need half-unit boundaries?
Worked example
Approximate P(X ≥ 1) for B(200,0.01) and compare with exact.
Show the worked solution
- The guide is met, with np = 2.
- Poisson gives 1 − e⁻² ≈ 0.8647.
- Exact binomial gives 1 − 0.99²⁰⁰ ≈ 0.8660.
Answer Approximate 0.8647; exact 0.8660.
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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.
For B(200,0.01), calculate Poisson and exact binomial P(X ≥ 1).
Give me a hint
Use zero-count complements separately in the two models.
Compare my reasoning
- Matching parameter is two and guide is met.
- Poisson gives 1 − e⁻² ≈ 0.8647.
- Exact gives 1 − 0.99²⁰⁰ ≈ 0.8660.
Approximate 0.8647; exact 0.8660.
Look for these in your work
- Check suitability.
- Compute exact complement independently.
For B(150,0.02), approximate P(2) and compare the exact mass.
Give me a hint
Use parameter three; nCr belongs only to the exact formula.
Compare my reasoning
- n > 50 and np = 3 < 5.
- Po(3) gives 4.5e⁻³ ≈ 0.2240.
- Exact 150C2 × 0.02² × 0.98¹⁴⁸ ≈ 0.2248.
Approximate 0.2240; exact 0.2248.
Look for these in your work
- Use the correct parameter.
- Keep the model formulae distinct.
For B(200,0.99), estimate at least 199 successes using rare failures and compare exact.
Give me a hint
At most one failure is the original requested event.
Compare my reasoning
- Failures match Po(2).
- Estimate P(failures ≤ 1) = 3e⁻² ≈ 0.4060.
- Exact = 200 × 0.01 × 0.99¹⁹⁹ + 0.99²⁰⁰ ≈ 0.4046.
Approximate 0.4060; exact 0.4046.
Look for these in your work
- Translate the event.
- Label each probability by model.
Common mistakes
- Label estimates separately.
- Translate success events into failures correctly.
- Similar bars do not imply identical support.
What can you explain now?
Do Po(2) and B(200,0.01) have identical support?
Compare with the explanation
No; Poisson continues beyond 200.
They are approximately matched, not identical distributions.
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Make it stick
Check suitability, calculate the discrete event, label the estimate and compare exact working when accuracy matters.
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.