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A Level · Cambridge (CIE) · 9709

Combine two datasets by restoring and adding moments

Weight means by counts, reconstruct second totals and include the spread between group centres.

Section 5.1 · Representation of data

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

  • Calculate descriptive first and second moments.
  • Decode any shifted or scaled totals into common units.

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

Quick readiness check

Groups have counts 4 and 6 with means 10 and 20. Their combined mean is…

What you will learn

  • Combine up to two compatible datasets using totals or means and standard deviations.
  • Explain why averaging group standard deviations is invalid.

Add counts and totals, not unweighted summaries

Group A has n = 4, mean 10 and descriptive standard deviation 2. Its sum is 40 and squared sum is 4(2² + 10²) = 416. B has n = 6, mean 20 and standard deviation 3, so sum 120 and squared sum 6(3² + 20²) = 2454. Combined n = 10, sum 160 and squared sum 2870; mean is 16 and variance 287 − 16² = 31, giving standard deviation √31 ≈ 5.568.

The combined mean is (nA meanA + nB meanB)/(nA + nB). The combined squared total uses n(variance + mean²) for each group. Do not use the unweighted mean 15 when counts differ. Standard deviations cannot simply be averaged: dispersion between group centres also contributes to the combined spread.

Compatibility and grouped estimates matter

If two groups have equal mean 5, counts 3 and 2 and descriptive standard deviations 1 and 2, the combined variance is (3(1²) + 2(2²))/5 = 11/5, giving standard deviation √(11/5). The between-mean contribution vanishes only because the means are equal. Even here, averaging the standard deviations is wrong; variance, not standard deviation, enters the second moment.

Use the same measurement units, definition of descriptive variance and coding before combining. If moments came from class midpoints, the combined result remains an estimate. Counts cannot be added to combine incompatible populations without explaining the purpose, and pooled descriptive summaries do not by themselves establish population-level inference or eliminate differences in measurement conditions.

Pause and explain

For n = 4, mean 10 and descriptive SD 2, what is Σx²?

Put the idea to work

Worked example

Combine A (n 4, mean 10, descriptive SD 2) and B (n 6, mean 20, descriptive SD 3) in the same units.

Show the worked solution
  1. Recover A totals 40, 416 and B totals 120, 2454 from n mean and n(SD² + mean²).
  2. Add to n = 10, Σx = 160, Σx² = 2870; combined mean = 16.
  3. Combined variance = 287 − 256 = 31; SD = √31 ≈ 5.568.

Answer Combined mean 16 and descriptive standard deviation √31 ≈ 5.568.

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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

Combine A (n 4, mean 10, descriptive SD 2) and B (n 6, mean 20, descriptive SD 3) in the same units.

Give me a hint

Restore each first and second total before adding.

Compare my reasoning
  1. A totals are 40 and 416; B totals are 120 and 2454.
  2. Combined n = 10, sum 160 and squared sum 2870, giving mean 16.
  3. Variance = 2870/10 − 16² = 31 and SD = √31 ≈ 5.568.

Combined n 10, mean 16, variance 31 and SD √31 ≈ 5.568.

Look for these in your work

  • Weight all observations through counts.
  • Include squared means when reconstructing moments.
2 · Independent

Two groups share mean 5 and have (n, descriptive SD) = (3,1) and (2,2). Find combined mean and SD.

Give me a hint

Equal means remove the between-centre term, but variances still need count weights.

Compare my reasoning
  1. Their first totals are 15 and 10; combined mean is 25/5 = 5.
  2. Squared totals are 3(1 + 25) = 78 and 2(4 + 25) = 58.
  3. Combined variance = 136/5 − 25 = 11/5; SD = √(11/5) ≈ 1.483.

Combined mean 5; variance 11/5; SD √(11/5) ≈ 1.483.

Look for these in your work

  • Weight variances by group counts when means match.
  • Do not average standard deviations.
3 · Transfer

A has three observations coded y = x − 10 with Σy = 3 and Σy² = 5. B has two observations coded z = x − 20 with Σz = 2 and Σz² = 4. Combine in original x units.

Give me a hint

The different origins must be undone before their totals are added.

Compare my reasoning
  1. A gives Σx = 33 and Σx² = 5 + 20(3) + 100(3) = 365.
  2. B gives Σx = 42 and Σx² = 4 + 40(2) + 400(2) = 884.
  3. Combined n = 5, sum 75 and squared sum 1249; mean 15, variance 1249/5 − 225 = 124/5, SD √(124/5) ≈ 4.980.

Combined mean 15; variance 124/5; SD √(124/5) ≈ 4.980 in original units.

Look for these in your work

  • Decode each group with its own origin.
  • Add compatible original moments rather than different coded totals.

Common mistakes

  • Weight group means by their counts.
  • Recover and add squared totals, not standard deviations.
  • Combine units and coding consistently; preserve grouped-estimate status.
Recall without your notes

What can you explain now?

Why can the combined standard deviation exceed both group standard deviations?

Compare with the explanation

The difference between group means adds spread in the pooled data.

Within-group spreads alone omit the separation of the groups’ centres.

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Make it stick

Restore n, sum and squared sum in common units, add them, and calculate the pooled descriptive mean and spread.

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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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