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

Discrete and binomial distributions

Use probability weights and recognise the assumptions behind a binomial model.

Statistics 1 · Discrete and binomial revision · 5.4

Probability & Statistics 1 (Paper 5) · Mathematics 9709, 2026–2027. This short revision overview checks weighted means and binomial assumptions; it does not cover the full section 5.4 pathway, including variance and geometric distributions. Written answers are self-checked, not officially marked.

Before you begin

  • Multiply probabilities for independent events and use a complement.
  • Multiply a value by its probability, including when the value is negative.

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

Quick readiness check

A table lists all possible values of X as 0, 2 and 5, with probabilities 0.3, 0.4 and 0.2. What prevents it from being a valid complete probability distribution?

What you will learn

  • Calculate an expected value.
  • Use a binomial model for repeated trials.

Expected value is a weighted mean

For a discrete random variable, list each possible value and its probability. Probabilities must be non-negative and sum to one. The expected value is the sum of value multiplied by probability; it need not be a possible outcome.

An expectation describes a long-run average under repeated use of the model. It does not predict the exact result of the next trial.

Check the binomial assumptions

A binomial variable counts successes in a fixed number n of independent trials, each with the same success probability p and two classified outcomes. When sampling without replacement from a small population, constant p and independence usually fail.

Exactly zero successes means every trial fails, giving (1−p)ⁿ. At least one success is the complement, 1−(1−p)ⁿ.

E(X)=ΣxP(X=x); P(X=r)=C(n,r)pʳ(1−p)ⁿ⁻ʳ

Sources for this lesson

Pause and explain

A bag contains 6 red and 4 blue counters. Two counters are drawn at random without replacement, and X counts the red counters. Which conclusion is correct?

Put the idea to work

Worked example

X takes values 0,1,3 with probabilities 0.2,0.5,0.3. Find E(X).

Show the worked solution
  1. Check the probabilities: 0.2+0.5+0.3=1.
  2. Multiply values by probabilities: 0,0.5,0.9.
  3. Add the weighted values.

Answer 1.4

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

X takes values −1, 1 and 4 with probabilities 0.2, 0.5 and 0.3. Find E(X) and explain whether it must be an outcome.

Give me a hint

Check the probability total, then keep the negative sign in the weighted sum.

Compare my reasoning
  1. The probabilities are non-negative and total 1.
  2. Weight every value: E(X) = (−1)(0.2) + (1)(0.5) + (4)(0.3) = −0.2 + 0.5 + 1.2.
  3. E(X) = 1.5. It is a model average, so it need not be one of the possible outcomes −1, 1 and 4.

E(X) = 1.5; it is not a possible outcome, and that is allowed.

Look for these in your work

  • Check that the table is valid.
  • Include the negative weighted term.
  • Interpret expectation as an average rather than a prediction of the next result.
2 · Independent

Five independent trials each have success probability 0.20. Find the probability of at least one success.

Give me a hint

The complement contains just one event: no successes.

Compare my reasoning
  1. The number of successes is B(5, 0.20): five fixed independent trials, with the same success probability.
  2. No successes means all five trials fail, so P(X = 0) = (0.80)^5 = 0.32768.
  3. Subtract from 1: P(X ≥ 1) = 1 − 0.32768 = 0.67232.

0.67232.

Look for these in your work

  • Use the no-success event as the complement.
  • Use the failure probability 0.80 in all five factors.
  • Keep the exact probability under the stated model.
3 · Transfer

A bag has 3 red and 2 blue counters. Compare three random draws with replacement and thorough mixing after each draw against three draws without replacement. In each case find the probability of no red counters, and decide whether the red count is binomial.

Give me a hint

Track how many blue counters remain; do not assume both sampling rules give independent trials.

Compare my reasoning
  1. With replacement and mixing, every draw has red probability 3/5 independently. The red count is B(3, 3/5), and no red has probability (2/5)^3 = 8/125.
  2. Without replacement, no red in three draws would require three blue counters, but the bag contains only two. Its probability is 0.
  3. Without replacement the conditional probabilities change, so an exact binomial model is not justified. Both counts are still discrete.

With replacement: 8/125 = 0.064, binomial. Without replacement: 0, not binomial.

Look for these in your work

  • State the sampling mechanism before choosing a model.
  • Use the available number of blue counters to recognise an impossible event.
  • Separate discrete values from binomial assumptions.

Common mistakes

  • Do not average the possible values without probability weights.
  • The expected value does not have to be an integer.
Recall without your notes

What can you explain now?

For four independent trials with success probability 0.25, find the probability of no successes.

Compare with the explanation

0.31640625

All four trials must fail, so the probability is 0.75⁴.

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

From memory, explain why probability weights matter for E(X), then name the assumptions you must check before using a binomial formula. Revisit the section 5.4 pathway for variance, geometric models and further probability problems.

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.