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A Level · Cambridge (CIE) · 9709

Interpret geometric expectation and choose attempt thresholds

Distinguish average waiting time from a probability guarantee and find the first integer deadline meeting a stated target.

Section 5.4 · Discrete random variables

Statistics 1 · Discrete random variables · 5.4
Exam scope and assessment notes

Probability & Statistics 1 (Paper 5, AS & A Level) · Mathematics 9709, 2026–2027, section 5.4. A probability table needs non-negative probabilities summing to one. E(X) is a probability-weighted mean; Var(X) = E(X²) − [E(X)]², with squared units. A binomial model B(n,p) counts successes in a fixed non-negative integer number n of independent two-outcome trials with a constant success probability p. A geometric model Geo(p) counts the trial of the first success, starting at 1, with independent trials and a constant 0 < p ≤ 1. Formula proofs are not required. The expectation of the geometric distribution is required; its variance is not required by section 5.4. Normal approximations belong to section 5.5 and are not used here. Exact model probabilities do not establish real-world independence or constant rates without evidence. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Calculate geometric tail probabilities.
  • Use logarithms and check inequality directions.

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

Quick readiness check

What is E(T) for Geo(1/5) under the first-success convention?

What you will learn

  • Use E(T) = 1/p under the first-success convention.
  • Find and verify an integer attempt threshold for a specified success probability.

An average is not a guaranteed deadline

For T ~ Geo(p) counting the first-success trial from one, E(T) = 1/p. At p = 1/5 the expected waiting time is five trials. A particular experiment may finish on trial one or much later; the expectation is the long-run average under repeated independent experiments with the same model. Formula proofs and geometric variance are not required by section 5.4.

If the expected first-success trial is exactly four, then p = 1/4. The chance of success by trial two is 1 − (3/4)² = 7/16. The chance by trial four is 175/256, about 68.36%, rather than 100% or 90%. The alternative failure-count variable T − 1 has mean 1/p − 1, so its convention would change an inferred probability.

Verify both sides of an integer threshold

With p = 1/4, a success-by-m probability of at least 90% requires 1 − (3/4)ᵐ ≥ 0.90, or (3/4)ᵐ ≤ 0.10. Taking logarithms and dividing by the negative ln(3/4) reverses the inequality: m ≥ ln(0.10)/ln(3/4), approximately 8.004. Therefore the first possible integer is nine, not eight.

Check the neighbours with unrounded values: by eight trials the probability is 0.8998870849609375, still below 0.90; by nine it is 0.9249153137207031, above the target. Rounding the eight-trial probability to 0.90 before comparison would choose the wrong threshold. For 0 < p < 1, no finite number of attempts guarantees success with probability one, even though eventual success has probability one under the ideal infinite-trial model.

Pause and explain

For Geo(1/4), what is the minimum number of trials giving at least a 90% chance of success?

Put the idea to work

Worked example

T ~ Geo(1/4). Find the minimum integer m with P(T ≤ m) ≥ 0.90.

Show the worked solution
  1. Require (3/4)ᵐ ≤ 0.10.
  2. Divide logarithms carefully to get m ≥ ln(0.10)/ln(3/4) ≈ 8.004, so test m = 9.
  3. The eight-trial probability is 0.8998870849609375 and the nine-trial probability is 0.9249153137207031.

Answer Minimum m = 9.

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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

T ~ Geo(1/5). Find E(T) and explain what it means.

Give me a hint

Take the reciprocal of p and interpret it over repeated experiments.

Compare my reasoning
  1. E(T) = 1/(1/5) = 5 trials.
  2. The variable includes the successful trial, rather than counting only failures.
  3. Five is the long-run mean waiting time under the model, not a guaranteed trial in one experiment.

E(T) = 5 trials; an average, not a guaranteed finishing time.

Look for these in your work

  • Use the first-success convention.
  • Distinguish expectation from a probability deadline.
2 · Independent

A geometric first-success variable has exact expectation 4. Find p and P(T ≤ 2).

Give me a hint

Infer p from the reciprocal mean, then complement two failures.

Compare my reasoning
  1. 1/p = 4 gives p = 1/4.
  2. P(T > 2) = (3/4)² = 9/16.
  3. P(T ≤ 2) = 1 − 9/16 = 7/16.

p = 1/4; P(T ≤ 2) = 7/16.

Look for these in your work

  • Infer p using the correct variable convention.
  • Use a cumulative event rather than exactly two trials.
3 · Transfer

T ~ Geo(1/4). Find the minimum integer m giving P(T ≤ m) ≥ 0.90 and explain why the mean is insufficient.

Give me a hint

Solve the failure-tail inequality and compare the candidate with its preceding integer.

Compare my reasoning
  1. (3/4)ᵐ ≤ 0.10 gives m ≥ ln(0.10)/ln(3/4) ≈ 8.004.
  2. At m = 8 the success probability is 0.8998870849609375; at m = 9 it is 0.9249153137207031.
  3. Choose m = 9. The mean is four trials, but success by four is only 175/256, so it does not meet the 90% target.

Minimum m = 9; the mean of four trials is not a 90% deadline.

Look for these in your work

  • Reverse the inequality when dividing by a negative logarithm.
  • Check the unrounded probability at both neighbouring integers.

Common mistakes

  • Expected waiting time is not a guaranteed completion time.
  • The first-success mean differs from the mean number of failures.
  • Do not round a near-target probability before choosing the integer threshold.
Recall without your notes

What can you explain now?

Does E(T) = 4 mean success is certain by trial four?

Compare with the explanation

No; for p = 1/4 that probability is only 175/256.

An average trial number and a cumulative probability answer different questions.

After trying it yourself, choose your next review. This is your self-assessment.

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Make it stick

Use 1/p for the mean first-success trial; use cumulative probabilities and neighbouring integers for a reliability threshold.

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.

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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.

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