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A Level · Cambridge (CIE) · 9709

Correct totals and infer missing observations consistently

Repair both moments when a value is corrected and use independent totals to check an inferred observation.

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

  • Use descriptive moment formulas and combined totals.
  • Solve simple numerical equations for an unknown value.

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

Quick readiness check

Correcting one existing value from 12 to 8 changes the observation count how?

What you will learn

  • Update first and second totals for a corrected or missing observation.
  • Check count, moment consistency and rounding before accepting a result.

A correction changes the squared total separately

Five observations were summarised by Σx = 40 and Σx² = 360, but a recorded 12 should have been 8. Keep n = 5 and correct the sum to 40 − 12 + 8 = 36. Correct the squared sum to 360 − 12² + 8² = 280, not by subtracting (12 − 8)². The corrected mean is 7.2, variance 280/5 − 7.2² = 4.16 and SD = 2√26/5 ≈ 2.040.

If an extra observation is added, count increases as well as both totals; if an observation is removed, all three change. For 2, 4, 4, 6, adding 9 gives n = 5, sum 25 and squared sum 153. The new mean is 5, variance 153/5 − 25 = 28/5 and SD √(28/5). A correction in place is different from adding a new measurement.

Use the second total as an independent check

Six observations have sum 30 and squared sum 190. Five known values 2, 3, 4, 5, 6 have sum 20 and squared sum 90, so the missing value from the first total is 10. Its square is 100, agreeing with 190 − 90. The full mean is 5 and descriptive variance 190/6 − 25 = 20/3. If the supplied squared total were 180 instead, the inferred 10 would not agree; do not force incompatible exact data into one answer.

Moment formulas require Σx² ≥ (Σx)²/n, because descriptive variance cannot be negative. Rounded published means or SDs can introduce small inconsistencies; use exact supplied totals when available and explain rounding limits. Do not clamp an appreciably negative variance to zero or claim a unique missing value if the supplied information does not determine it.

Pause and explain

A squared sum 360 includes a wrong 12 that should be 8. The corrected squared sum is…

Put the idea to work

Worked example

For n = 5, sum 40 and squared sum 360, correct one observation from 12 to 8 and find the new mean and SD.

Show the worked solution
  1. The corrected sum is 36; the corrected squared sum is 360 − 144 + 64 = 280.
  2. Mean = 36/5 = 7.2; variance = 280/5 − (36/5)² = 104/25.
  3. SD = √104/5 = 2√26/5 ≈ 2.040; n remains 5.

Answer Corrected mean 7.2 and SD 2√26/5 ≈ 2.040; totals 36 and 280.

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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

Correct n = 5, Σx = 40, Σx² = 360 when one recorded 12 should be 8. Find mean and descriptive SD.

Give me a hint

Keep count fixed, but repair each moment with its own operation.

Compare my reasoning
  1. Corrected totals are 36 and 280, with n still 5.
  2. Mean = 7.2 and variance = 280/5 − 7.2² = 104/25 = 4.16.
  3. SD = 2√26/5 ≈ 2.040.

Totals 36 and 280; mean 7.2; variance 4.16; SD 2√26/5 ≈ 2.040.

Look for these in your work

  • Replace both the value and its square.
  • Keep the count unchanged and retain working precision.
2 · Independent

Add observation 9 to raw data 2, 4, 4, 6. Find the new count, totals, mean and descriptive SD.

Give me a hint

Adding a measurement changes n as well as both totals.

Compare my reasoning
  1. Old totals 16 and 72 become 25 and 153, with n = 5.
  2. Mean = 25/5 = 5 and variance = 153/5 − 25 = 28/5.
  3. SD = √(28/5) ≈ 2.366.

n 5; totals 25 and 153; mean 5; variance 28/5; SD √(28/5) ≈ 2.366.

Look for these in your work

  • Increase the count by one.
  • Add 9², not just 9, to the second total.
3 · Transfer

Six observations have exact sum 30 and squared sum 190. Five known values are 2, 3, 4, 5, 6. Infer the sixth, find variance and explain what would change if the squared sum were instead 180.

Give me a hint

Use the first total, then verify its square with the independent second total.

Compare my reasoning
  1. Known sum 20 leaves missing x = 10; known squared sum 90 leaves 100, agreeing with 10².
  2. Mean = 5 and variance = 190/6 − 25 = 20/3.
  3. A squared total 180 would leave 90 instead of 100 and conflict with the exact first total; the data would be inconsistent.

Missing value 10; variance 20/3; squared total 180 would be inconsistent with the exact sum.

Look for these in your work

  • Check the inferred observation against both totals.
  • Report incompatible exact data rather than adjusting variance arbitrarily.

Common mistakes

  • Correct x² by removing the old square and adding the new square.
  • Distinguish replacing an observation from adding one.
  • Check a missing value against both exact totals.
Recall without your notes

What can you explain now?

If exact totals imply a negative descriptive variance, should you take its absolute value?

Compare with the explanation

No; check the supplied data and arithmetic for inconsistency.

A negative variance is impossible for real observations; changing its sign hides the problem.

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

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Make it stick

Repair count, sum and squared sum according to the event, then check non-negative variance and agreement of independent totals.

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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