Draw cumulative frequencies and estimate quartiles and proportions
Plot counts at upper boundaries, read cumulative ranks and make explicit the within-class assumptions behind percentile estimates.
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
- Accumulate frequencies in ordered classes.
- Read and interpolate points on two numerical axes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where should cumulative count 16 after the 10–20 class be plotted?
What you will learn
- Construct a cumulative frequency graph from grouped counts.
- Estimate quartiles, percentiles and proportions using consistent interpolation.
Accumulate counts at upper class boundaries
For classes [0,10), [10,20), [20,40), [40,50) with counts 4, 12, 8, 6, cumulative counts are 4, 16, 24, 30. Plot (0,0), (10,4), (20,16), (40,24), (50,30), using time on the horizontal axis and cumulative frequency vertically. The graph must not decrease and its final count must be 30. Plot upper boundaries, not class midpoints.
A drawn curve gives approximate readings. These exercises join points by straight segments, which estimates a uniform spread inside each class. For rank r in a class with lower boundary L, width w, frequency f and cumulative count c before the class, estimate x = L + (r − c)w/f. This is interpolation, not an exact recovery of the individual observations.
Read ranks and counts in the correct direction
The estimated median uses rank 15 in the 10–20 class: 10 + (15 − 4)10/12 = 115/6 ≈ 19.17 minutes. Q1 rank 7.5 gives 155/12 ≈ 12.92; Q3 rank 22.5 gives 145/4 = 36.25. Thus estimated IQR is 70/3 ≈ 23.33 minutes. The 80th percentile uses rank 24, at boundary 40 minutes.
At time 15, the interpolated cumulative count is 4 + (5/10)12 = 10, giving an estimated fraction below 15 of 10/30 = 1/3. At time 30 the cumulative count is 16 + (10/20)8 = 20, so the fraction above 30 is (30 − 20)/30 = 1/3. Between two cutoffs subtract their cumulative counts. At exact class boundaries counts can follow directly from the grouped table; inside classes they are estimates. The continuous-time model does not resolve tied recorded values at a cutoff.
Pause and explain
There are 30 journeys and estimated cumulative count 20 at 30 minutes. What proportion is estimated above 30?
Worked example
Estimate the median and 80th percentile from cumulative points (0,0), (10,4), (20,16), (40,24), (50,30), using linear interpolation.
Show the worked solution
- There are 30 observations; the median rank is 15 and the 80th-percentile rank is 24.
- Rank 15 lies between counts 4 and 16, giving 10 + 11(10)/12 = 115/6 minutes.
- Rank 24 is at upper boundary 40 minutes; this follows directly from the cumulative class count.
Answer Estimated median 115/6 ≈ 19.17 minutes; 80th percentile 40 minutes under the interpolation convention.
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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.
For boundaries 0, 10, 20, 40, 50 minutes and frequencies 4, 12, 8, 6, give cumulative plotting points and estimate the median by straight-line interpolation.
Give me a hint
First form cumulative counts and identify the class containing rank 15.
Compare my reasoning
- Plot (0,0), (10,4), (20,16), (40,24), (50,30).
- The median rank 30/2 = 15 lies in [10,20).
- Estimate median = 10 + (15 − 4)10/12 = 115/6 ≈ 19.17 minutes.
Points (0,0), (10,4), (20,16), (40,24), (50,30); estimated median 115/6 ≈ 19.17 minutes.
Look for these in your work
- Use upper boundaries and a zero starting point.
- State the linear within-class interpolation assumption.
Use the same grouped table to estimate Q1, Q3, IQR and the 80th percentile.
Give me a hint
Use cumulative ranks 7.5, 22.5 and 24.
Compare my reasoning
- Q1 = 10 + (7.5 − 4)10/12 = 155/12 ≈ 12.92 minutes.
- Q3 = 20 + (22.5 − 16)20/8 = 145/4 = 36.25 minutes.
- IQR = 70/3 ≈ 23.33 minutes; rank 24 gives the 80th percentile at 40 minutes.
Estimated Q1 155/12, Q3 145/4, IQR 70/3 minutes; 80th percentile 40 minutes.
Look for these in your work
- Select the correct class for each rank.
- Keep the estimated nature of within-class quartiles explicit.
Under linear interpolation in that table, estimate proportions below 15, above 30 and between 15 and 30 minutes. Can you call the within-class counts exact?
Give me a hint
Interpolate the two cumulative counts, then subtract as needed.
Compare my reasoning
- Estimated cumulative counts at 15 and 30 are 10 and 20.
- Below 15: 10/30; above 30: (30 − 20)/30; between: (20 − 10)/30; all are 1/3.
- They are estimates because neither cutoff is an original class boundary; raw tied measurements are not resolved.
All three proportions are estimated as 1/3; the within-class counts are not exact raw counts.
Look for these in your work
- Use a difference of cumulative counts for an interval.
- Distinguish interpolation from actual measurements.
Common mistakes
- Plot cumulative counts at upper boundaries, beginning at the lowest boundary with zero.
- Use n/4, n/2 and 3n/4 for these grouped graph ranks.
- Counts above a value are n minus the cumulative count, not the count below it.
What can you explain now?
Does an interpolated cumulative count of 20 at 30 minutes prove exactly 20 raw journeys were shorter than 30?
Compare with the explanation
No; 30 lies inside a grouped class, so this is an estimate.
The original table does not disclose individual positions inside the 20–40 class.
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Make it stick
Accumulate at boundaries, read a rank across and down, and use cumulative differences for interval proportions.
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