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A Level · Cambridge (CIE) · 9709

Read conditional probabilities from two-way tables

Build disjoint cells from overlapping totals and choose the conditioning row or column as the denominator.

Statistics 1 · Probability · 5.3
Exam scope and assessment notes

Probability & Statistics 1 (Paper 5, AS & A Level) · Mathematics 9709, 2026–2027, section 5.3. Define the sample space and state its probability assumptions before counting. Fair independent dice give equiprobable ordered pairs; their sums are not equiprobable. Random selection means each specified individual is equally likely under the stated sampling model. Without replacement, later branch probabilities change; with replacement and independent mixing, they remain the same. A′ is a complement, A ∩ B means both, A ∪ B means either or both, and P(A|B) means A given B with P(B) > 0. Mutually exclusive and independent are different properties. The explicit general addition formula is not required by this section; explain overlap through disjoint regions. Conditional probabilities may use P(A|B) = P(A ∩ B)/P(B). Numerical models describe the stipulated experiment, not automatically a real population. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Use intersections and complements.
  • Identify the denominator in a conditional probability.

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

Quick readiness check

A totals 48 and both totals 30. How many are A-only?

What you will learn

  • Complete a two-way count table from event totals.
  • Calculate conditional probabilities and distinguish sample descriptions from population claims.
A two-way table of 120 recorded students uses rows A and not-A and columns B and not-B. Cells are 30, 18, 30, 42 respectively; row totals are 48 and 72, column totals 60 and 60. P(A|B) is 30/60 = 1/2, while P(B|A) is 30/48 = 5/8. These probabilities describe a uniform choice from the recorded group.Use the given row or column totalBNot BTotalANot ATotal3018483042726060120P(A | B) = 30/60 = 1/2P(B | A) = 30/48 = 5/8Recorded group; uniform choice
A two-way table of 120 recorded students uses rows A and not-A and columns B and not-B. Cells are 30, 18, 30, 42 respectively; row totals are 48 and 72, column totals 60 and 60. P(A|B) is 30/60 = 1/2, while P(B|A) is 30/48 = 5/8. These probabilities describe a uniform choice from the recorded group.

Overlapping totals must be separated into cells

Among 120 recorded students, 48 are in club A, 60 in club B and 30 in both. The disjoint cells are both 30, A-only 18, B-only 30 and neither 42. A row/column table has totals A 48, not-A 72, B 60 and not-B 60. Check all cells add to 120 before taking ratios.

For a student chosen uniformly from this recorded group, P(A|B) = 30/60 = 1/2. Conversely P(B|A) = 30/48 = 5/8. The same intersection is divided by different given-event totals. P(not-A|B) = 30/60 = 1/2 complements P(A|B) within the B group, not within the entire population.

Condition within the correct column or row

In an eighty-person recorded group, both has 24, A-only 16, B-only 8 and neither 32. The B column totals 32, so P(A|B) = 24/32 = 3/4. The not-B column totals 48, so P(A|not-B) = 16/48 = 1/3. Do not use total A = 40 as the denominator for either of these questions.

Uniform selection from a recorded table supports exact probabilities for that finite group. Using observed proportions to predict another population requires a representative sampling model. A voluntary poll of 40 respondents with 18 bus travellers describes a bus proportion 18/40 = 0.45 in that respondent group; it does not automatically establish the same proportion for all students.

Pause and explain

120 students: 48 in A, 60 in B, 30 in both. What is P(A|B)?

Put the idea to work

Worked example

Among 120 students, 48 belong to A, 60 to B and 30 to both. Choose a student uniformly. Find P(A|B) and P(B|A).

Show the worked solution
  1. The shared cell has 30 students.
  2. Conditioning on B gives denominator 60, so P(A|B) = 1/2.
  3. Conditioning on A gives denominator 48, so P(B|A) = 5/8.

Answer P(A|B) = 1/2; P(B|A) = 5/8.

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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

Among 120 students, 48 belong to A, 60 to B and 30 to both. Choose uniformly from the group. Find P(A|B) and P(B|A).

Give me a hint

Use the shared count with the appropriate club total each time.

Compare my reasoning
  1. The shared numerator is 30 in both calculations.
  2. P(A|B) = 30/60 = 1/2.
  3. P(B|A) = 30/48 = 5/8.

1/2 and 5/8 respectively.

Look for these in your work

  • Read the condition direction.
  • Explain the different denominators.
2 · Independent

A table of 80 people has both 24, A-only 16, B-only 8 and neither 32. Choose uniformly. Find P(A|B) and P(A|not-B).

Give me a hint

Add the cells in each conditioning column before calculating its ratio.

Compare my reasoning
  1. The B total is 24 + 8 = 32, so P(A|B) = 24/32 = 3/4.
  2. The not-B total is 16 + 32 = 48, so P(A|not-B) = 16/48 = 1/3.
  3. The different denominators correspond to different information being given.

3/4 and 1/3 respectively.

Look for these in your work

  • Complete the conditioning totals.
  • Keep the A-only cell distinct from all A members.
3 · Transfer

A voluntary travel poll has 40 respondents: 18 use a bus, 12 walk and 10 cycle, with one recorded mode per person. Find the bus probability for a uniformly chosen respondent and assess a claim that it is exactly the same for every student population.

Give me a hint

A uniform choice from respondents is different from representative sampling of all students.

Compare my reasoning
  1. The respondent categories sum to 40.
  2. A uniformly chosen respondent uses the bus with probability 18/40 = 0.45.
  3. Voluntary participation can be unrepresentative, so the population claim is unsupported by these counts alone.

0.45 for the recorded respondent group; the general population claim is unsupported.

Look for these in your work

  • State which finite group the probability describes.
  • Identify the missing representative-sampling assumption.

Common mistakes

  • Marginal totals include the shared cell.
  • Use the given row or column total, not the whole table.
  • A recorded-group proportion does not establish a representative population probability.
Recall without your notes

What can you explain now?

Why does P(A|B) + P(A′|B) equal one?

Compare with the explanation

The A and not-A cells partition the B conditioning group.

Both fractions use the same positive B total as denominator.

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

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Make it stick

Complete the disjoint cells, check totals, and divide by the given row or column rather than an unrelated marginal.

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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