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A Level · Cambridge (CIE) · 9709

Data summaries

Choose an average that fits the data and interpret it with an appropriate measure of spread.

Probability & Statistics 1 · Data summaries revision · 5.1
Exam scope and assessment notes

Probability & Statistics 1 (Paper 5) · Mathematics 9709, 2026–2027. This overview practises raw and discrete-frequency summaries and interpreting basic box plots; it does not cover all of section 5.1, standard deviation or grouped/coded data. Written solutions use self-check criteria, not an official mark scheme. Specialist accuracy review has not been completed. The diagram uses separate illustrative data, not the values in the practice tasks. For quartiles in these tasks, exclude the overall median from an odd-sized list, then take the median of each half. Box-plot whiskers here extend to the minimum and maximum; no outlier-exclusion rule is applied.

Before you begin

  • Order numerical data and distinguish a value from its position.
  • Multiply values by frequencies and interpret units.

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

Quick readiness check

Six recorded times are 18, 4, 11, 7, 15, 9 minutes. What is their median?

What you will learn

  • Find the median of ordered data.
  • Explain why a mean may be affected by outliers.
Two basic minimum-to-maximum box plots use a shared time scale. A summary is minimum 2, Q1 3, median 6, Q3 9.5, maximum 11 minutes. B summary is 1, 4, 5, 7, 13 minutes. B has a lower median and smaller IQR, 3 versus 6.5 minutes, but a larger range, 12 versus 9 minutes. Numerical distances are drawn to the common scale; whisker lengths are not frequency counts.Compare centre and named spreadA2369.511B14571302468101214Time / minutesIQR: A 6.5; B 3 minutes
Two basic minimum-to-maximum box plots use a shared time scale. A summary is minimum 2, Q1 3, median 6, Q3 9.5, maximum 11 minutes. B summary is 1, 4, 5, 7, 13 minutes. B has a lower median and smaller IQR, 3 versus 6.5 minutes, but a larger range, 12 versus 9 minutes. Numerical distances are drawn to the common scale; whisker lengths are not frequency counts.

Order before selecting a position

The median divides ordered data into two halves. For an odd number n of values, it is at position (n+1)/2. For an even number, average the two central values. A position is an index into the sorted list, not the numerical answer.

The mean uses every value and can move strongly when an extreme observation is added. The median depends on order and is often more resistant to an outlier.

A centre does not describe everything

Two data sets can have the same mean but very different variability. The range is maximum minus minimum; other measures describe spread around the centre. When comparing distributions, discuss both typical value and spread, and use the context's units.

For a frequency table, treat frequencies as repeated observations. The total number of observations is the sum of frequencies, not the number of rows.

Sources for this lesson

Pause and explain

Delivery-time samples C and D have (median, IQR, range) of (8, 2, 30) and (10, 5, 12) minutes respectively. Which comparison is justified?

Put the idea to work

Worked example

Find the median of 8, 3, 15, 9, 7, 10.

Show the worked solution
  1. Sort the data: 3,7,8,9,10,15.
  2. There are six values; select positions 3 and 4.
  3. Average 8 and 9.

Answer 8.5

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

A discrete table records waiting times of 4, 7 and 12 minutes with frequencies 3, 4 and 1 respectively. Find the number of observations, mean, median and range. Explain why averaging the three row values is inappropriate.

Give me a hint

Use frequencies as repeats. Locate the central positions using cumulative counts.

Compare my reasoning
  1. There are 3 + 4 + 1 = 8 observations. The total waiting time is 4×3 + 7×4 + 12×1 = 52 minutes.
  2. The mean is 52/8 = 6.5 minutes. The unweighted row average 23/3 would give each distinct time equal weight despite their different frequencies.
  3. The ordered list is 4, 4, 4, 7, 7, 7, 7, 12. Positions 4 and 5 are both 7, so the median is 7 minutes. The range is 12 − 4 = 8 minutes.

n = 8; mean 6.5 minutes; median 7 minutes; range 8 minutes.

Look for these in your work

  • Use the total frequency as the number of observations.
  • Calculate a frequency-weighted sum before dividing.
  • Locate both central observations and retain time units.
2 · Independent

Two seven-journey samples have times A: 5, 6, 7, 7, 7, 8, 44 and B: 4, 6, 8, 9, 10, 12, 14 minutes. Calculate each mean, median, IQR and range. Compare typical time and spread, and explain why the means and medians give different rankings. Use the stated quartile rule.

Give me a hint

Report the middle-half spread separately from the full range. Keep the large delay in the discussion.

Compare my reasoning
  1. A has total 84, mean 12 and median 7 minutes. Excluding the median, the halves are 5, 6, 7 and 7, 8, 44: Q1 = 6, Q3 = 8, IQR = 2 minutes. Its range is 44 − 5 = 39 minutes.
  2. B has total 63, mean 9 and median 9 minutes. Its half-list medians are Q1 = 6 and Q3 = 12, giving IQR = 6 minutes. Its range is 14 − 4 = 10 minutes.
  3. A has a lower median and a smaller middle-half spread, but a larger range. Its 44-minute delay lifts the mean above B’s. Median and IQR describe the typical time and middle spread without hiding that operationally important delay. Neither sample proves one service is always faster or establishes its long-run performance.

A: mean 12, median 7, IQR 2, range 39 minutes. B: mean 9, median 9, IQR 6, range 10 minutes.

Look for these in your work

  • Calculate all four summaries using one explicit quartile convention.
  • Compare centre and both measures of spread in context.
  • Explain the effect of 44 without deleting it or generalising beyond the samples.
3 · Transfer

A learner says that identical basic box plots guarantee identical means. Test the claim using scores P: 2, 4, 5, 6, 7, 8, 12 and Q: 2, 4, 6, 6, 8, 8, 12 marks. Calculate both five-number summaries under the stated quartile rule, then both means. Identify information a box plot does not retain.

Give me a hint

A five-number summary records selected positions, not the sum of every observation.

Compare my reasoning
  1. Each sorted list has seven values and median 6. Excluding the median, both have Q1 = 4 and Q3 = 8. Their minimum and maximum are 2 and 12, so both summaries are (2, 4, 6, 8, 12) marks.
  2. P totals 44 marks and Q totals 46 marks. Their means are 44/7 and 46/7 marks, approximately 6.29 and 6.57. They are unequal despite identical five-number summaries.
  3. A basic box plot does not retain every internal observation or its total. These explicit datasets disprove the claim; a mean cannot in general be recovered from the five-number summary alone.

Same summary (2, 4, 6, 8, 12); different means 44/7 and 46/7 marks.

Look for these in your work

  • Check both medians and both half-list medians.
  • Compute each mean from all seven scores.
  • Use the unequal means as a counterexample, not a claim that box plots are useless.

Common mistakes

  • Do not find a median from an unsorted list.
  • The median of an even-sized set need not be one of its observations.
Recall without your notes

What can you explain now?

Which changes more when 100 replaces 10 in 2,4,6,8,10: mean or median?

Compare with the explanation

The mean

The median stays 6. The mean rises from 6 to 24.

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

Explain why median position is not median value, why a frequency-table mean needs weights, and how a smaller IQR can coexist with a larger range. State one feature that a basic box plot cannot determine.

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.

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