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A Level · Cambridge (CIE) · 9709

Draw cumulative frequencies and estimate quartiles and proportions

Plot counts at upper boundaries, read cumulative ranks and make explicit the within-class assumptions behind percentile estimates.

Statistics 1 · Representation of data · 5.1
Exam scope and assessment notes

Probability & Statistics 1 (Paper 5, AS & A Level) · Mathematics 9709, 2026–2027, section 5.1. Label variables, units, class boundaries and diagram keys. In these raw-data quartile exercises, use the median of each ordered half, excluding the overall median when the number of observations is odd; follow any convention supplied in an examination question. Cumulative-frequency quartiles use ranks n/4 and 3n/4, with a median rank n/2. Grouped-data interpolation and midpoint moments are estimates, not recovered raw measurements. Histogram area represents frequency, with height proportional to frequency density. Use descriptive variance with divisor n, not the sample-estimator divisor n − 1. Coded and combined totals must refer to the same units and compatible coding before addition. Retain unrounded working and distinguish variance units from standard-deviation units. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Accumulate frequencies in ordered classes.
  • Read and interpolate points on two numerical axes.

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

Quick readiness check

Where should cumulative count 16 after the 10–20 class be plotted?

What you will learn

  • Construct a cumulative frequency graph from grouped counts.
  • Estimate quartiles, percentiles and proportions using consistent interpolation.
Cumulative frequency points are (0,0), (10,4), (20,16), (40,24), (50,30), plotted at upper time boundaries. Straight segments assume uniform spread inside each class. The median rank 15 is read across to the 10–20 segment and down to estimated time 115/6, about 19.17 minutes. The 80th-percentile rank 24 is at boundary 40 minutes. These graph readings do not recover individual within-class measurements.Read an estimated time from rankCumulative frequency / observations01015202430Median rank 15010204050Time / minutesEstimated median: 19.17 minutesStraight segments assume uniform spread
Cumulative frequency points are (0,0), (10,4), (20,16), (40,24), (50,30), plotted at upper time boundaries. Straight segments assume uniform spread inside each class. The median rank 15 is read across to the 10–20 segment and down to estimated time 115/6, about 19.17 minutes. The 80th-percentile rank 24 is at boundary 40 minutes. These graph readings do not recover individual within-class measurements.

Accumulate counts at upper class boundaries

For classes [0,10), [10,20), [20,40), [40,50) with counts 4, 12, 8, 6, cumulative counts are 4, 16, 24, 30. Plot (0,0), (10,4), (20,16), (40,24), (50,30), using time on the horizontal axis and cumulative frequency vertically. The graph must not decrease and its final count must be 30. Plot upper boundaries, not class midpoints.

A drawn curve gives approximate readings. These exercises join points by straight segments, which estimates a uniform spread inside each class. For rank r in a class with lower boundary L, width w, frequency f and cumulative count c before the class, estimate x = L + (r − c)w/f. This is interpolation, not an exact recovery of the individual observations.

Read ranks and counts in the correct direction

The estimated median uses rank 15 in the 10–20 class: 10 + (15 − 4)10/12 = 115/6 ≈ 19.17 minutes. Q1 rank 7.5 gives 155/12 ≈ 12.92; Q3 rank 22.5 gives 145/4 = 36.25. Thus estimated IQR is 70/3 ≈ 23.33 minutes. The 80th percentile uses rank 24, at boundary 40 minutes.

At time 15, the interpolated cumulative count is 4 + (5/10)12 = 10, giving an estimated fraction below 15 of 10/30 = 1/3. At time 30 the cumulative count is 16 + (10/20)8 = 20, so the fraction above 30 is (30 − 20)/30 = 1/3. Between two cutoffs subtract their cumulative counts. At exact class boundaries counts can follow directly from the grouped table; inside classes they are estimates. The continuous-time model does not resolve tied recorded values at a cutoff.

Pause and explain

There are 30 journeys and estimated cumulative count 20 at 30 minutes. What proportion is estimated above 30?

Put the idea to work

Worked example

Estimate the median and 80th percentile from cumulative points (0,0), (10,4), (20,16), (40,24), (50,30), using linear interpolation.

Show the worked solution
  1. There are 30 observations; the median rank is 15 and the 80th-percentile rank is 24.
  2. Rank 15 lies between counts 4 and 16, giving 10 + 11(10)/12 = 115/6 minutes.
  3. Rank 24 is at upper boundary 40 minutes; this follows directly from the cumulative class count.

Answer Estimated median 115/6 ≈ 19.17 minutes; 80th percentile 40 minutes under the interpolation convention.

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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

For boundaries 0, 10, 20, 40, 50 minutes and frequencies 4, 12, 8, 6, give cumulative plotting points and estimate the median by straight-line interpolation.

Give me a hint

First form cumulative counts and identify the class containing rank 15.

Compare my reasoning
  1. Plot (0,0), (10,4), (20,16), (40,24), (50,30).
  2. The median rank 30/2 = 15 lies in [10,20).
  3. Estimate median = 10 + (15 − 4)10/12 = 115/6 ≈ 19.17 minutes.

Points (0,0), (10,4), (20,16), (40,24), (50,30); estimated median 115/6 ≈ 19.17 minutes.

Look for these in your work

  • Use upper boundaries and a zero starting point.
  • State the linear within-class interpolation assumption.
2 · Independent

Use the same grouped table to estimate Q1, Q3, IQR and the 80th percentile.

Give me a hint

Use cumulative ranks 7.5, 22.5 and 24.

Compare my reasoning
  1. Q1 = 10 + (7.5 − 4)10/12 = 155/12 ≈ 12.92 minutes.
  2. Q3 = 20 + (22.5 − 16)20/8 = 145/4 = 36.25 minutes.
  3. IQR = 70/3 ≈ 23.33 minutes; rank 24 gives the 80th percentile at 40 minutes.

Estimated Q1 155/12, Q3 145/4, IQR 70/3 minutes; 80th percentile 40 minutes.

Look for these in your work

  • Select the correct class for each rank.
  • Keep the estimated nature of within-class quartiles explicit.
3 · Transfer

Under linear interpolation in that table, estimate proportions below 15, above 30 and between 15 and 30 minutes. Can you call the within-class counts exact?

Give me a hint

Interpolate the two cumulative counts, then subtract as needed.

Compare my reasoning
  1. Estimated cumulative counts at 15 and 30 are 10 and 20.
  2. Below 15: 10/30; above 30: (30 − 20)/30; between: (20 − 10)/30; all are 1/3.
  3. They are estimates because neither cutoff is an original class boundary; raw tied measurements are not resolved.

All three proportions are estimated as 1/3; the within-class counts are not exact raw counts.

Look for these in your work

  • Use a difference of cumulative counts for an interval.
  • Distinguish interpolation from actual measurements.

Common mistakes

  • Plot cumulative counts at upper boundaries, beginning at the lowest boundary with zero.
  • Use n/4, n/2 and 3n/4 for these grouped graph ranks.
  • Counts above a value are n minus the cumulative count, not the count below it.
Recall without your notes

What can you explain now?

Does an interpolated cumulative count of 20 at 30 minutes prove exactly 20 raw journeys were shorter than 30?

Compare with the explanation

No; 30 lies inside a grouped class, so this is an estimate.

The original table does not disclose individual positions inside the 20–40 class.

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Make it stick

Accumulate at boundaries, read a rank across and down, and use cumulative differences for interval proportions.

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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.

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