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A Level · Cambridge (CIE) · 9709

Find raw-data centre, quartiles and spread

Order measurements, select the right positions and distinguish a typical value from overall and middle-half variation.

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 data including repeated values.
  • Calculate sums and averages.

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

Quick readiness check

For eight ordered values, which positions determine the median?

What you will learn

  • Calculate mean, median, mode, range and IQR for raw data.
  • Explain sensitivity to extremes and the stated raw quartile convention.

Position and frequency answer different questions

For ordered values 2, 3, 3, 4, 8, 9, 10, 11 minutes, the mean is 50/8 = 6.25 and median is (4 + 8)/2 = 6 minutes. The mode is 3 because it appears most often; it need not equal either centre measure. The range is 11 − 2 = 9 minutes.

A median uses ordered positions, not half of the sum. With an even count average the two middle values; with an odd count use the single middle value. A dataset can have several modes, or no uniquely most frequent value. Do not automatically choose the smallest value when frequencies tie. Report units alongside every measure.

Specify the raw quartile convention

In this pathway’s raw-data questions, take medians of the lower and upper ordered halves, excluding the overall median for an odd count. The lower half 2, 3, 3, 4 gives Q1 = 3; the upper half 8, 9, 10, 11 gives Q3 = 9.5. The IQR is 9.5 − 3 = 6.5 minutes. Follow a convention provided in an exam question rather than silently switching definitions.

The mean uses every value and can move strongly when an extreme value changes. The median and IQR focus on positions and can remain unchanged. Replacing 11 with 101 in this eight-value dataset raises the mean to 140/8 = 17.5 and the range to 99, while median stays 6; Q3 remains 9.5 under the stated convention. A robust summary is useful for typical experience, but extreme values can still be substantively important.

Pause and explain

If Q1 = 3 and Q3 = 9.5 minutes, what is the IQR?

Put the idea to work

Worked example

For 2, 3, 3, 4, 8, 9, 10, 11 minutes, calculate mean, median, mode, range and IQR using medians of the two halves.

Show the worked solution
  1. Sum = 50, so mean = 6.25; middle pair 4 and 8 gives median 6; the mode is 3.
  2. Range = 11 − 2 = 9; lower-half median is Q1 = 3 and upper-half median Q3 = 9.5.
  3. IQR = 9.5 − 3 = 6.5 minutes.

Answer Mean 6.25; median 6; mode 3; range 9; Q1 3, Q3 9.5, IQR 6.5 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

Find mean, median and mode of 2, 3, 3, 4, 8, 9, 10, 11 minutes.

Give me a hint

Use the sum for the mean, middle pair for the median and greatest frequency for mode.

Compare my reasoning
  1. The sum is 50 and count 8, giving mean 6.25 minutes.
  2. The ordered middle values 4 and 8 give median 6 minutes.
  3. The value 3 appears twice and all others once, so mode is 3 minutes.

Mean 6.25, median 6 and mode 3 minutes.

Look for these in your work

  • Do not confuse the middle position with the mean.
  • Use repetition to identify the mode.
2 · Independent

For ordered values 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 marks, exclude the overall median before taking medians of the two halves. Find Q1, median, Q3, IQR and range.

Give me a hint

There are five values below 6 and five above it.

Compare my reasoning
  1. The overall median is the sixth value, 6 marks.
  2. Lower half 1–5 has median 3; upper half 7–11 has median 9.
  3. IQR = 9 − 3 = 6 and range = 11 − 1 = 10 marks.

Q1 3, median 6, Q3 9; IQR 6, range 10 marks.

Look for these in your work

  • Exclude the overall median under the stated convention.
  • Keep middle-half and whole-range spreads separate.
3 · Transfer

Replace 11 with 101 in 2, 3, 3, 4, 8, 9, 10, 11 minutes. Compare the new mean, median, range and IQR with the originals using the same quartile rule.

Give me a hint

The sum rises by 90 but the middle positions stay fixed.

Compare my reasoning
  1. The new sum is 140, so mean = 17.5 minutes; the middle pair still gives median 6.
  2. The new range is 101 − 2 = 99 minutes.
  3. Q1 remains 3 and Q3 remains 9.5; IQR remains 6.5 minutes.

Mean 17.5, median 6, range 99 and IQR 6.5 minutes; the median and IQR are unchanged.

Look for these in your work

  • Recalculate the sum and extremes.
  • Explain positional stability under this specific change rather than claiming universal immunity.

Common mistakes

  • Order values before finding positional summaries.
  • IQR is Q3 − Q1, not maximum minus minimum.
  • State the quartile convention and keep units.
Recall without your notes

What can you explain now?

If only the largest value 11 becomes 101 here, must the median increase?

Compare with the explanation

No; its middle positions still contain 4 and 8.

The median stays 6 even though the sum, mean and range change.

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

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Make it stick

Use sum for mean, positions for median and quartiles, frequencies for mode, and the correct pair of endpoints for each 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.

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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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