Check a Poisson approximation to a binomial count
Apply the large-n small-p syllabus guide and relabel rare failures when successes are common.
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
- Recall binomial assumptions and moments.
- Use the complement count n − X.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What parameter matches B(n,p) in the Poisson approximation?
What you will learn
- Check n > 50 and np < 5 approximately for Po(np).
- Translate success events into a rare failure count.
Match the rare-count mean
For B(n,p), use Po(np) when n is large and p small. Cambridge gives n > 50 and np < 5 approximately as a practical guide. B(100,0.02) meets it and matches Po(2); its exact variance is 1.96, while the approximate variance is two.
B(40,0.02) has np = 0.8 but n is not greater than fifty. It fails the stated guide; that does not prove every numerical estimate would be inaccurate. Exact binomial calculation remains available. The guide is not a universal error guarantee.
Change the variable before changing the model
For X ~ B(200,0.99), success mean 198 is not rare. Define failures Y = 200 − X ~ B(200,0.01), then approximate Y by Po(2). Writing X ~ Po(2) would describe the wrong event and mean.
At least 199 successes becomes Y ≤ 1. Translate the event before calculation. No continuity correction is needed for this discrete-to-discrete approximation; half-unit boundaries are for continuous normal areas representing integer cells.
Pause and explain
For X ~ B(200,0.99), which failure event Y = 200 − X means X ≥ 199?
Worked example
Check a Poisson approximation to B(100,0.02) and compare variances.
Show the worked solution
- n = 100 > 50 and np = 2 < 5, with small p.
- Use approximate Po(2), mean and variance two.
- Exact binomial variance is 100 × 0.02 × 0.98 = 1.96.
Answer Po(2); approximate variance 2 versus exact 1.96.
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.
Check a Poisson approximation for B(100,0.02) and compare moments.
Give me a hint
Calculate np and np(1 − p) before naming the estimate.
Compare my reasoning
- n = 100 > 50 and np = 2 < 5, with small p.
- Po(2) has mean and variance two.
- Exact binomial mean is two and variance 1.96.
Suitable by guide: Po(2), approximate variance 2 versus exact 1.96.
Look for these in your work
- Check both guide criteria.
- Distinguish exact and approximate variance.
Does B(40,0.02) meet the guide, and what method gives exact probabilities?
Give me a hint
Check n, not only the expected count.
Compare my reasoning
- np = 0.8 < 5.
- n = 40 does not exceed fifty.
- The guide is not met; exact binomial calculation remains available.
It fails the stated n guide; use exact binomial probabilities when required.
Look for these in your work
- Check every criterion.
- Avoid treating the guide as an error theorem.
For X ~ B(200,0.99), choose a rare-count approximation and translate X ≥ 199.
Give me a hint
Define failures before applying the small-probability guide.
Compare my reasoning
- Y = 200 − X ~ B(200,0.01).
- It meets the guide and matches Po(2).
- X ≥ 199 is Y ≤ 1.
Approximate failures by Po(2); calculate Y ≤ 1.
Look for these in your work
- Name the variable approximated.
- Translate the inequality.
Common mistakes
- Check n as well as the expected count.
- Rare failures are a different variable from successes.
- Do not continuity-correct discrete-to-discrete calculations.
What can you explain now?
Which count is rare when success probability is 0.99?
Compare with the explanation
Failures, with probability 0.01.
Relabel the count and event before using its small parameter.
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Make it stick
Identify the rare event, check n and expected count, then match Po(np) and translate any relabelled event.
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.