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

Construct an approximate population proportion interval

Use successes divided by sample size and the estimated binomial standard error under suitable large-sample conditions.

Statistics 2 · Sampling and estimation · 6.4
Exam scope and assessment notes

Probability & Statistics 2 (Paper 6, A Level) · Mathematics 9709, 2026–2027, section 6.4. Paper 6 builds on Paper 5 and assumes Paper 3 calculus. Distinguish population parameters, sample statistics and repeated-sample random variables. Random selection needs a relevant frame and does not automatically remove undercoverage or non-response. Particular named sampling methods are not required. Sample-mean formulae here assume independent observations from one population with finite variance: E(X̄) = μ, Var(X̄) = σ²/n and SE = σ/√n. A mean of independent normal observations is normal; the CLT gives an approximate normal mean for an adequate large independent sample, not normal individual data. No universal sample-size cutoff guarantees that approximation. Unbiased sample variance uses n − 1, with n > 1. Mean intervals use the normal model with known variance or a large-sample approximation; a small sample with unknown variance does not justify the z method taught here. Proportion intervals use the large-sample normal approximation with adequate success and failure counts; extreme or sparse counts may invalidate it. Quoted two-sided critical values are 1.645 for 90%, 1.960 for 95% and 2.576 for 99%; retain other working unrounded. Confidence describes repeated-sampling coverage, not posterior probability for a fixed realised interval, individual observations or a guarantee. A narrower interval does not repair a biased sample. Hypothesis testing belongs to section 6.5. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending.

Before you begin

  • Use a binomial success model.
  • Calculate an estimated standard error.

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

Quick readiness check

What is p̂ for 72 successes among 200 observations?

What you will learn

  • Estimate a population proportion.
  • Calculate a large-sample approximate confidence interval.
72 successes among 200 independent observations give p̂ = 0.36, estimated SE approximately 0.03394 and 95% normal margin approximately 0.06652. Endpoints are approximately 0.2935,0.4265. The displayed parameter axis is a partial 0.2–0.5 range, not probability density or full support. The normal interval needs a suitable large-sample binary model and adequate outcome counts.Estimate a population success proportion72/200 = 0.360.250.360.45Estimated SE ≈ 0.0339495% margin ≈ 0.06652Approximate interval [0.2935,0.4265]Partial parameter axis · model needed
72 successes among 200 independent observations give p̂ = 0.36, estimated SE approximately 0.03394 and 95% normal margin approximately 0.06652. Endpoints are approximately 0.2935,0.4265. The displayed parameter axis is a partial 0.2–0.5 range, not probability density or full support. The normal interval needs a suitable large-sample binary model and adequate outcome counts.

Estimate p from binary outcomes

With 72 successes in 200 independent observations, p̂ = 72/200 = 0.36. Estimated SE is √(0.36 × 0.64/200) ≈ 0.0339411. The approximate 95% interval p̂ ± 1.960SE is [0.2935,0.4265], retaining unrounded working until the endpoints.

The target p is the population probability of the consistently defined success event. The observed p̂ is a sample statistic. Do not confuse the proportion with the success count or use a mean-measurement SD formula.

Use the large-sample approximation honestly

For 120 successes in 400 observations, p̂ = 0.3 and the approximate 95% interval is [0.2551,0.3449]. This normal interval assumes the sampling model and enough expected success and failure outcomes; observed counts provide a practical adequacy check, not a universal accuracy guarantee.

Express endpoints as proportions or percentages consistently. The first interval corresponds to about 29.35% through 42.65%, not 0.2935% through 0.4265%. A narrow computed interval does not repair voluntary-response bias.

Pause and explain

Which estimated SE belongs to the large-sample proportion interval?

Put the idea to work

Worked example

Find a 95% interval for 72 successes from 200 observations.

Show the worked solution
  1. p̂ = 0.36.
  2. Estimated SE = √(0.36 × 0.64/200).
  3. Margin ≈ 0.0665246; add and subtract.

Answer Approximately [0.2935,0.4265].

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

Find an approximate 95% p interval for 72 successes in 200 suitable independent observations.

Give me a hint

Estimate p and its SE, then use z = 1.960.

Compare my reasoning
  1. p̂ = 0.36.
  2. SE ≈ 0.0339411 and margin ≈ 0.0665246.
  3. Endpoints ≈ 0.2935 and 0.4265.

Approximately [0.2935,0.4265].

Look for these in your work

  • Use both success and failure proportions.
  • State approximation.
2 · Independent

Find the corresponding 95% interval for 120 successes in 400 observations.

Give me a hint

Apply the same formula to the new proportion and n.

Compare my reasoning
  1. p̂ = 0.3.
  2. SE = √(0.21/400); margin ≈ 0.0449092.
  3. Endpoints ≈ 0.2551 and 0.3449.

Approximately [0.2551,0.3449].

Look for these in your work

  • Recalculate both components.
  • Keep unrounded working.
3 · Transfer

Express the first interval as percentages and describe its target.

Give me a hint

Multiply proportion endpoints by one hundred.

Compare my reasoning
  1. Lower endpoint is about 29.35%.
  2. Upper endpoint is about 42.65%.
  3. The interval estimates the population success proportion, not an individual binary result.

About 29.35%–42.65% for the population success proportion.

Look for these in your work

  • Use percentage units correctly.
  • State the population parameter.

Common mistakes

  • Divide success count by n.
  • Use p̂(1 − p̂) inside SE.
  • Keep proportion and percentage units consistent.
Recall without your notes

What can you explain now?

Is p̂ the fixed population parameter?

Compare with the explanation

No; it varies with the sample.

It estimates the unknown p.

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

A proportion interval uses p̂ ± z√(p̂(1 − p̂)/n), with a justified large-sample binary model.

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.