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A Level · Cambridge (CIE) · 9709

Compare two raw datasets with back-to-back stems

Use shared stems and two directional keys to compare raw distributions without reversing their place value or erasing repeats.

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

  • Build a single ordered stem-and-leaf diagram.
  • Find a median from an ordered list.

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

Quick readiness check

For tens stems on the left, what is leaf 4 beside stem 2?

What you will learn

  • Construct ordered back-to-back stem-and-leaf diagrams.
  • Recover values and make numerical comparisons on a shared scale.
Back-to-back stem-and-leaf diagram of A times 12, 14, 14, 18, 21, 23, 26, 29 and B times 11, 16, 18, 19, 22, 24, 24, 28 minutes. A is left of shared tens stems 1 and 2; B is right. Left leaves decrease from left to right, with the smallest nearest the stem; right leaves increase. Directional keys identify A leaf 2 beside stem 1 as 12 and B stem 1 beside leaf 1 as 11 minutes. A median is 19.5 and B median 20.5 minutes; both ranges are 17 minutes.Shared stems, directional keysA · leftStemB · right184421689296312448Left: 2 | 1 = 12 minutesRight: 1 | 1 = 11 minutesMedians: A 19.5; B 20.5Both ranges: 17 minutes
Back-to-back stem-and-leaf diagram of A times 12, 14, 14, 18, 21, 23, 26, 29 and B times 11, 16, 18, 19, 22, 24, 24, 28 minutes. A is left of shared tens stems 1 and 2; B is right. Left leaves decrease from left to right, with the smallest nearest the stem; right leaves increase. Directional keys identify A leaf 2 beside stem 1 as 12 and B stem 1 beside leaf 1 as 11 minutes. A median is 19.5 and B median 20.5 minutes; both ranges are 17 minutes.

Share the stem and order leaves away from it

Group A times are 12, 14, 14, 18, 21, 23, 26, 29; Group B times are 11, 16, 18, 19, 22, 24, 24, 28 minutes. Put A on the left and B on the right of shared stems 1 and 2. The left rows, read from left to right, are 8 4 4 2 and 9 6 3 1; the right rows are 1 6 8 9 and 2 4 4 8. On either side, the smallest leaf is nearest the stem.

Label both groups and use keys 2 | 1 on the left means 12 minutes, while 1 | 1 on the right means 11 minutes. The first part of the left key is the leaf and the second the stem. Both datasets must use the same units and compatible precision. Do not read the left row as a separate decimal number or discard its repeated leaves.

Compare values, not the visual row length alone

Recovering A in order gives median (18 + 21)/2 = 19.5 minutes. B has median (19 + 22)/2 = 20.5 minutes. A has a lower median by 1 minute in these observations. Their ranges are 29 − 12 = 17 and 28 − 11 = 17 minutes, so the overall spreads by range are equal.

Equal ranges do not force equal IQRs, means or distribution shapes. Each group here has eight observations; a diagram with different counts needs separate medians from its own count. The comparison describes these measurements, without establishing why the times differ. A shared stem helps comparison but does not replace numerical evidence.

Pause and explain

Which left row is correctly ordered for 12, 14, 14, 18 beside stem 1?

Put the idea to work

Worked example

Construct a back-to-back diagram for the two eight-time datasets above and compare their medians.

Show the worked solution
  1. Use A-left rows 8 4 4 2 | 1 and 9 6 3 1 | 2; B-right rows 1 | 1 6 8 9 and 2 | 2 4 4 8.
  2. Provide a direction-aware key on each side and retain all eight leaves per group.
  3. A median is 19.5 minutes and B median 20.5 minutes; A is lower by 1 minute.

Answer A median 19.5 minutes; B median 20.5 minutes; A is lower by 1 minute.

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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

Write Group A = 12, 14, 14, 18 and B = 11, 16, 18, 19 on shared stem 1, A-left and B-right, with keys.

Give me a hint

Left leaves decrease from left to right; right leaves increase.

Compare my reasoning
  1. The shared row is A: 8 4 4 2 | 1 | 1 6 8 9 :B.
  2. Left key 2 | 1 = 12 minutes; right key 1 | 1 = 11 minutes.
  3. Each side has four leaves, including A’s two values at 14.

8 4 4 2 | 1 | 1 6 8 9; four leaves per group, with directional time keys.

Look for these in your work

  • Order both sides consistently.
  • Retain the duplicate and explain place value on both sides.
2 · Independent

A times are 12, 14, 14, 18, 21, 23, 26, 29; B times are 11, 16, 18, 19, 22, 24, 24, 28 minutes. Compare medians and ranges.

Give me a hint

Both groups have eight values, so each median is the average of its fourth and fifth values.

Compare my reasoning
  1. A median = (18 + 21)/2 = 19.5; B median = (19 + 22)/2 = 20.5 minutes.
  2. A range = 29 − 12 = 17; B range = 28 − 11 = 17 minutes.
  3. A has a lower median by 1 minute, while the ranges are equal.

Medians A 19.5, B 20.5 minutes; both ranges 17 minutes.

Look for these in your work

  • Use each group’s ordered middle pair.
  • Compare centre and spread separately.
3 · Transfer

A diagram uses stem 2 with leaves 0 5 9 on the right under key 2 | 0 = 2.0 seconds. Another group records 20, 25, 29 seconds. Can those entries share the same key as if they were comparable? Explain.

Give me a hint

A shared key fixes measurement scale, not just matching digits.

Compare my reasoning
  1. The first leaves encode 2.0, 2.5 and 2.9 seconds.
  2. The second group’s values are ten times as large; the same leaves under that key would misstate them.
  3. Use compatible place value and units for both groups, or a different representation with a shared numerical scale.

No: 2.0, 2.5, 2.9 seconds differ from 20, 25, 29 seconds; align the scale and key first.

Look for these in your work

  • Decode the supplied key before comparing.
  • Do not infer equivalence from matching leaf digits.

Common mistakes

  • On the left, the smallest leaf is nearest the stem.
  • Give a key for each direction.
  • Equal ranges do not imply identical distributions.
Recall without your notes

What can you explain now?

What is the left entry leaf 8 beside stem 1 under a tens-and-units key?

Compare with the explanation

18, not 81.

The shared stem still represents tens regardless of which side holds the leaf.

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

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Make it stick

Recover values with the directional keys, count each group separately, then compare numerical centre and spread.

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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