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A Level · Cambridge (CIE) · 9709

Find normal percentiles and central probability limits

Convert upper-tail or central-area requirements into cumulative probabilities before using an inverse normal value.

Statistics 1 · Normal distribution · 5.5
Exam scope and assessment notes

Probability & Statistics 1 (Paper 5, AS & A Level) · Mathematics 9709, 2026–2027, section 5.5. N(μ,σ²) uses variance as its second parameter, with σ > 0; probabilities are areas, not density heights. Show the full standardisation Z = (X − μ)/σ. The MF19 table gives Φ(z) = P(Z ≤ z), with negative values found by symmetry. Model sketches and shaded probabilities may be required. Probability answers use quoted four-decimal table values where supplied; calculations with non-tabulated z use unrounded standardisation and numerical normal probabilities, so table interpolation can differ slightly in the last digits. Inverse examples use the quoted three-decimal MF19 critical values and are approximate. A binomial normal approximation requires both np > 5 and n(1 − p) > 5, and a continuity correction. It remains approximate, including for integer probability thresholds. A continuous normal variable has zero probability at a single exact value; a discrete binomial count does not. Sample means, the Central Limit Theorem and hypothesis testing belong to Statistics 2 and are not covered here. Model assumptions need evidence in real applications. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Read cumulative and critical normal tables.
  • Standardise a normal measurement.

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

Quick readiness check

If P(X > c) = 0.10, which cumulative probability locates c?

What you will learn

  • Find a percentile using x = μ + zσ.
  • Translate upper tails and central coverage into the correct cumulative quantiles.
A normal curve for N(100,225) shades the lower 90% up to its approximate ninetieth percentile. Using the quoted critical value z = 1.282 and standard deviation 15 gives c = 100 + 1.282 × 15 = 119.23. The remaining upper area is approximately 10%. The mean 100 and boundary 119.23 are marked; tails extend beyond the displayed curve.N(100, 225) · SD = 15100119.23Lower 90% · upper 10%(c − 100)/15 ≈ 1.282c ≈ 119.23Quoted critical value · approximate
A normal curve for N(100,225) shades the lower 90% up to its approximate ninetieth percentile. Using the quoted critical value z = 1.282 and standard deviation 15 gives c = 100 + 1.282 × 15 = 119.23. The remaining upper area is approximately 10%. The mean 100 and boundary 119.23 are marked; tails extend beyond the displayed curve.

A percentile is a cumulative boundary

For X ~ N(100,225), σ = 15. The 90th percentile c has P(X ≤ c) = 0.90, so use the MF19 critical value z ≈ 1.282. Then (c − 100)/15 ≈ 1.282, giving c ≈ 119.23. A requirement that only 10% exceed c uses the same cumulative probability 0.90.

The bottom 10th percentile instead has z ≈ −1.282 and c ≈ 100 − 1.282(15) = 80.77. Check the percentile’s side of the mean before calculating. Probability inputs strictly between zero and one give finite normal quantiles; cumulative zero and one do not describe finite cut-offs in an unbounded normal model.

A central probability leaves two tails

A central 95% interval leaves 5% outside, split as 2.5% in each tail. Its cumulative boundaries are 0.025 and 0.975, so the quoted critical values are −1.960 and 1.960. For Y ~ N(80,16), the limits are 80 ± 1.960(4), approximately 72.16 and 87.84.

Do not use z = 1.645 for a central 95% interval: that is the cumulative 95th-percentile boundary, leaving 5% on one side. Inverse values quoted to three decimals yield approximate limits, not exact identities for the underlying curve. Keep the chosen percentile, sign, units and standardisation visible in the working.

Pause and explain

Which upper cumulative probability belongs to a central 95% normal interval?

Put the idea to work

Worked example

X ~ N(100,225). Find c so that P(X > c) = 0.10 using z₀.₉₀ ≈ 1.282.

Show the worked solution
  1. The cumulative probability below c is 0.90.
  2. Write (c − 100)/15 ≈ 1.282.
  3. Solve c ≈ 100 + 15(1.282) = 119.23.

Answer c ≈ 119.23.

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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

X ~ N(100,225). Find the 90th percentile using z₀.₉₀ ≈ 1.282.

Give me a hint

Multiply the quoted standard normal quantile by SD and add the mean.

Compare my reasoning
  1. σ = 15 and (c − 100)/15 ≈ 1.282.
  2. Thus c ≈ 100 + 19.23.
  3. The 90th-percentile boundary is approximately 119.23, above the mean.

Approximately 119.23.

Look for these in your work

  • Use cumulative probability 0.90.
  • Show standardisation and the original measurement units.
2 · Independent

X ~ N(100,225). Find the bottom 10th percentile using the same positive critical value 1.282.

Give me a hint

Reflect the standard normal quantile before returning to X.

Compare my reasoning
  1. The lower tenth-percentile z value is approximately −1.282.
  2. (c − 100)/15 ≈ −1.282.
  3. Solve c ≈ 100 − 19.23 = 80.77.

Approximately 80.77.

Look for these in your work

  • Use a negative z for a lower percentile.
  • Check the answer is below the mean.
3 · Transfer

Y ~ N(80,16). Find central 95% limits using z₀.₉₇₅ ≈ 1.960.

Give me a hint

Leave 2.5% in each tail and use SD four.

Compare my reasoning
  1. The cumulative boundaries are 0.025 and 0.975.
  2. Their z values are approximately −1.960 and 1.960.
  3. The measurement limits are 80 ± 7.84, giving 72.16 and 87.84.

Approximately 72.16 to 87.84.

Look for these in your work

  • Split the excluded probability into equal tails.
  • Use the variance convention and state approximate limits.

Common mistakes

  • Convert an upper-tail probability before choosing a cumulative quantile.
  • Split the excluded probability between both tails for central coverage.
  • Quoted critical values yield approximate measurement limits.
Recall without your notes

What can you explain now?

Does central 95% use the cumulative 95th-percentile z value?

Compare with the explanation

No; its upper cumulative boundary is 97.5%.

The remaining 5% is divided equally between the lower and upper tails.

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Make it stick

Convert the required area to a cumulative quantile, check its sign, and solve the full standardisation for the original boundary.

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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