Use Poisson mean variance and standard deviation
Use equal population mean and variance and distinguish exact model moments from variable sample summaries.
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
- Distinguish variance from standard deviation.
- Read the exposure-specific parameter.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the SD of Po(9)?
What you will learn
- Use E(X) = Var(X) = λ and SD(X) = √λ.
- Distinguish population identities from sample observations.
One parameter gives both moments
For Po(9), mean and variance each equal nine, while SD = √9 = 3. Their numerical equality does not equate units: mean and SD use count units, variance squared count units. Proofs of these results are not required by the syllabus.
If SD is two, variance is four, hence λ = 4 and mean is four. Then P(0) = e⁻⁴ ≈ 0.0183. Using the SD directly as λ would give incorrect moments and probabilities.
Keep population and sample statements distinct
Exact population mean four and variance twelve cannot both describe a Poisson distribution, since both must equal λ. A sample mean four and sample variance twelve, however, may differ through sampling variation and do not logically prove a Poisson population impossible.
Even exact equal mean and variance are necessary properties, not a complete characterization of the distribution. Assess the mechanism, exposure, independence and data. Sample summaries can prompt review without certifying or rejecting a model on their own.
Pause and explain
Can exact population mean 4 and variance 12 describe a Poisson distribution?
Worked example
State mean, variance and SD of Po(9).
Show the worked solution
- The parameter is nine.
- Mean and variance each equal nine.
- The positive square root gives SD three.
Answer Mean 9; variance 9; SD 3.
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.
State mean, variance and SD for Po(9).
Give me a hint
Use λ for both moments, then its square root for SD.
Compare my reasoning
- Mean is nine.
- Variance is nine.
- SD is √9 = 3.
Mean 9; variance 9; SD 3.
Look for these in your work
- Use both moment results.
- Keep SD distinct.
A Poisson count has SD two. Find λ, mean, variance and P(0).
Give me a hint
Square SD before identifying the Poisson parameter.
Compare my reasoning
- Variance is four, hence λ = 4.
- Mean and variance each equal four.
- P(0) = e⁻⁴ ≈ 0.0183.
λ = 4; mean 4; variance 4; P(0) ≈ 0.0183.
Look for these in your work
- Infer parameter from variance.
- Use it in the mass formula.
Compare ‘population mean 4, variance 12’ with ‘sample mean 4, variance 12’. Which logically rules out a Poisson population?
Give me a hint
Distinguish exact model moments from random sample summaries.
Compare my reasoning
- Exact Poisson moments must both equal λ.
- The unequal exact population values conflict with that identity.
- Unequal sample summaries can reflect sampling variation and do not alone prove impossibility.
The exact population claim rules it out; the sample claim alone does not.
Look for these in your work
- Identify the type of each statement.
- Avoid overclaiming from sample summaries.
Common mistakes
- SD is √λ.
- Variance has squared units.
- Sample summaries are not exact population moments.
What can you explain now?
Does equal sample mean and variance prove a Poisson population?
Compare with the explanation
No; it does not determine the whole distribution.
Model assumptions still need suitable evidence.
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
Use λ for mean and variance, √λ for SD, and keep population identities separate from sample observations.
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.