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A Level · Cambridge (CIE) · 9709

Build ordered stem-and-leaf diagrams with an unambiguous key

Keep repeated measurements, empty stems and recorded precision visible instead of turning a display into a list of different values.

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

  • Order numerical values including repeats.
  • Understand place value and measurement units.

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

Quick readiness check

How many leaves are needed for 12, 14, 14, 18?

What you will learn

  • Construct and decode ordered stem-and-leaf diagrams.
  • Use decimal and split-stem keys without losing observations.
An ordered stem-and-leaf diagram preserves all eight times: 12, 14, 14, 18, 21, 23, 26, 29 minutes. Stem 1 has leaves 2, 4, 4, 8; stem 2 has 1, 3, 6, 9. Key 1 | 2 means 12 minutes. Each leaf is one observation, so both values at 14 remain.One leaf per recorded timeStemOrdered leaves1244821369Key: 1 | 2 = 12 minutes4 + 4 leaves = 8 observationsBoth 14-minute times are kept
An ordered stem-and-leaf diagram preserves all eight times: 12, 14, 14, 18, 21, 23, 26, 29 minutes. Stem 1 has leaves 2, 4, 4, 8; stem 2 has 1, 3, 6, 9. Key 1 | 2 means 12 minutes. Each leaf is one observation, so both values at 14 remain.

One leaf records one observation

For times 12, 14, 14, 18, 21, 23, 26 and 29 minutes, stem 1 has leaves 2 4 4 8 and stem 2 has leaves 1 3 6 9. The key 1 | 2 means 12 minutes. Keep both 4 leaves: two journeys took 14 minutes. Leaves on the right of a stem are written in increasing order.

Count the leaves to check that all eight observations are present. Include empty intermediate stems when necessary so a gap in the measurement range remains visible. For times 12, 14, 31, a stem 2 with no leaves makes the gap clear; skipping its row can make the scale harder to read. A diagram key must identify the numeric value and units, not just say that the leaves are ordered.

Precision and splitting change the key

For 2.1, 2.4, 2.4 and 3.0 seconds, use stems 2 and 3 with key 2 | 1 = 2.1 seconds. A leaf 0 in stem 3 represents 3.0, not 30. The same characters can encode different scales, so the key matters. Do not invent extra decimal precision beyond the measurements.

If a row is crowded, split each stem into two rows for leaves 0–4 and 5–9. For 20, 22, 25, 27 and 29, the lower split of stem 2 has 0 2 and the upper split has 5 7 9. Label the split convention. Repeated stems without an explanation can be misread as duplicate measurements or a changed place value.

Pause and explain

Under key 2 | 1 = 2.1 seconds, what is 3 | 0?

Put the idea to work

Worked example

Draw an ordered stem-and-leaf diagram for 12, 14, 14, 18, 21, 23, 26, 29 minutes.

Show the worked solution
  1. Use tens as stems 1 and 2 and units as leaves.
  2. Write 1 | 2 4 4 8 and 2 | 1 3 6 9; there are eight leaves including the repeat.
  3. Add key 1 | 2 = 12 minutes.

Answer 1 | 2 4 4 8; 2 | 1 3 6 9; key 1 | 2 = 12 minutes.

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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

Construct an ordered diagram for 12, 14, 14, 18, 21, 23, 26, 29 minutes and check its count.

Give me a hint

Use tens stems and preserve the duplicated 14.

Compare my reasoning
  1. Stem 1 has 2 4 4 8 and stem 2 has 1 3 6 9.
  2. The leaf count is 4 + 4 = 8, matching the raw dataset.
  3. Key 1 | 2 = 12 minutes makes the precision and unit explicit.

1 | 2 4 4 8; 2 | 1 3 6 9; 8 observations; key 1 | 2 = 12 minutes.

Look for these in your work

  • Keep both 14 observations.
  • Provide an interpretable key and count check.
2 · Independent

Display 2.1, 2.4, 2.4, 3.0 seconds using whole-number stems. Recover the largest value from your diagram.

Give me a hint

The leaves represent tenths rather than units.

Compare my reasoning
  1. Write 2 | 1 4 4 and 3 | 0.
  2. Use key 2 | 1 = 2.1 seconds; four leaves represent four measurements.
  3. The largest entry 3 | 0 is 3.0 seconds.

2 | 1 4 4; 3 | 0; key 2 | 1 = 2.1 seconds; largest 3.0 seconds.

Look for these in your work

  • Preserve the repeated 2.4.
  • Decode the zero leaf using the supplied precision.
3 · Transfer

Split stem 2 into leaves 0–4 and 5–9 for scores 20, 22, 25, 27, 29. Explain how to label the two rows.

Give me a hint

State which half of the leaf digits belongs to each repeated stem.

Compare my reasoning
  1. The 0–4 row is 2 | 0 2.
  2. The 5–9 row is 2 | 5 7 9, giving five leaves in total.
  3. Label both row ranges and key 2 | 5 = 25 marks so the repeated stem is intentional.

Stem 2 (0–4): 0 2; stem 2 (5–9): 5 7 9; key 2 | 5 = 25 marks.

Look for these in your work

  • Partition the leaf digits without overlap.
  • Explain the repeated stem and measurement units.

Common mistakes

  • Keep repeated leaves as repeated observations.
  • Put a value and units in the key.
  • Explain split stems and retain relevant empty rows.
Recall without your notes

What can you explain now?

What does 3 | 0 mean when the key is 2 | 1 = 2.1 seconds?

Compare with the explanation

3.0 seconds.

The leaf is the tenths digit under the supplied key.

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Make it stick

Order one leaf per observation and check the count, key, precision and any split or empty stems.

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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