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A Level · Cambridge (CIE) · 9709

Estimate mean and standard deviation from grouped intervals

Use each class midpoint for moment estimates while explaining why precise arithmetic cannot reconstruct the original measurements.

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 frequency-weighted first and second moments.
  • Find numerical interval midpoints.

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

Quick readiness check

What representative midpoint is used for [20,40)?

What you will learn

  • Estimate grouped mean and standard deviation using midpoint moments.
  • State the information lost by grouping and keep estimates labelled.

Use a representative midpoint in each class

For classes [10,20), [20,30), [30,40) marks with frequencies 2, 6, 2, use midpoints 15, 25, 35. n = 10, Σfm = 250 and Σfm² = 6650. Estimated mean is 25 and estimated variance is 665 − 625 = 40; estimated standard deviation is 2√10 ≈ 6.325 marks. The formula weights midpoint squares, not upper-boundary squares.

Replacing each unknown value by its class midpoint is a modelling assumption. The actual observations can lie elsewhere within their intervals, so neither the raw mean nor standard deviation is generally exact. Choosing narrower classes can reduce lost location information, but does not prove every estimate is closer in every dataset. Keep the estimate label even when a midpoint total happens to be an integer.

Unequal widths do not change frequency weighting

For times [0,10), [10,20), [20,40), [40,50) with frequencies 4, 12, 8, 6, use midpoints 5, 15, 30, 45. Then n = 30, Σfm = 710 and Σfm² = 22150. Estimated mean is 71/3 ≈ 23.67 minutes; estimated variance is 1604/9 and standard deviation 2√401/3 ≈ 13.35 minutes.

Frequencies weight the moments. Frequency densities are histogram heights and must first be converted to counts using class widths if only a density histogram is supplied. Averaging midpoints without their weights treats each class as equally frequent, which is a different dataset. Retain unrounded mean and second moment until the final square root.

Pause and explain

For n = 10, Σfm = 250 and Σfm² = 6650, the estimated variance is…

Put the idea to work

Worked example

Estimate mean and standard deviation for [10,20), [20,30), [30,40) marks with frequencies 2, 6, 2.

Show the worked solution
  1. Midpoints 15, 25, 35 give n = 10, Σfm = 250 and Σfm² = 6650.
  2. Estimated mean = 25 and variance = 6650/10 − 25² = 40.
  3. Estimated standard deviation = √40 = 2√10 ≈ 6.325 marks; individual within-class positions remain unknown.

Answer Estimated mean 25 marks; variance 40; standard deviation 2√10 ≈ 6.325 marks.

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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

Estimate mean and standard deviation for classes [10,20), [20,30), [30,40) with counts 2, 6, 2 marks.

Give me a hint

Use midpoint totals 250 and 6650 for ten observations.

Compare my reasoning
  1. Midpoints 15, 25, 35 give estimated mean 250/10 = 25 marks.
  2. Estimated variance = 6650/10 − 25² = 40.
  3. Estimated standard deviation = 2√10 ≈ 6.325 marks.

Estimated mean 25 and standard deviation 2√10 ≈ 6.325 marks.

Look for these in your work

  • Weight midpoint squares by frequencies.
  • Retain estimate language.
2 · Independent

For time classes [0,10), [10,20), [20,40), [40,50) and counts 4, 12, 8, 6, calculate midpoint totals and estimate mean and standard deviation.

Give me a hint

The unequal-width class [20,40) has midpoint 30 and count 8.

Compare my reasoning
  1. Midpoints 5, 15, 30, 45 give Σfm = 710, Σfm² = 22150, n = 30.
  2. Mean estimate = 71/3; variance estimate = 22150/30 − (71/3)² = 1604/9.
  3. Standard-deviation estimate = 2√401/3 ≈ 13.35 minutes.

Totals 710 and 22150; estimated mean 71/3 ≈ 23.67 and standard deviation 2√401/3 ≈ 13.35 minutes.

Look for these in your work

  • Use frequency 8 rather than density 0.4 as the third weight.
  • Keep unrounded moments in the variance calculation.
3 · Transfer

Two actual datasets each place two observations in [10,20) and two in [20,30). Dataset A is 10, 10, 20, 20; B is 19, 19, 29, 29. Compare their raw means with the common midpoint mean estimate.

Give me a hint

Both grouped tables match, but their observations sit at different class locations.

Compare my reasoning
  1. Both midpoint tables use 15 twice and 25 twice, giving estimated mean 20.
  2. A has raw mean 15 while B has raw mean 24.
  3. The common grouped estimate does not determine either exact raw mean.

Common midpoint mean estimate 20; actual means A 15 and B 24.

Look for these in your work

  • Verify both datasets satisfy the same interval counts.
  • Use the example to explain lost within-class information.

Common mistakes

  • Use midpoints and frequencies, not class endpoints or density weights.
  • Label mean and standard deviation as estimates.
  • Keep working precision before subtracting moments.
Recall without your notes

What can you explain now?

Does an estimated grouped mean of exactly 25 prove the raw mean was 25?

Compare with the explanation

No; individual positions inside each interval are unknown.

Exact midpoint arithmetic does not remove grouping uncertainty.

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

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Make it stick

Replace unknown within-class values by midpoints, weight by counts, calculate moments and preserve the estimate label.

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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