Higher: quartiles and interquartile range of discrete data
Higher: find quartiles and IQR from ordered discrete lists or frequency tables using an explicitly stated convention.
Exam scope and assessment notes
The first four lessons are shared Foundation work inherited by Higher; the last four are additional Higher spread, quartile and cumulative-frequency reading work. Keep the actual repeated observations and frequencies, state units, and distinguish an average from spread. Grouped midpoint means and within-class graph readings are estimates. The chosen discrete quartile rule is median of each half, excluding the overall middle observation when the count is odd; it is not claimed as a universal Pearson algorithm. Draw the horizontal and vertical guides yourself, use each graph’s own total, and retain a full plateau interval when an inverse is not unique. Fixed numerical agreement does not mark reasoning or award marks or mastery. Specialist and formal-assessment approval remain pending. This is the Issue 2 linear 4MA1 scope: November 2026/June 2027 entries and regional-centre eligibility differ from modular awards. Literal second-edition Book2 Handling Data4/5/6 and Hub tags guide navigation only; no Book1 6.2 tag or verified chapter/Hub body is inferred. Handling Data6 map208–222 conflicts with proof chapter207/summary221/next222; Handling Data4 map39–60 extends beyond summary59 before next61. Graphical representation6.1 and probability6.3 remain separate.
Before you begin
- Order a complete list with repetitions.
- Find medians of even and odd lists.
- Subtract signed and fractional numbers.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Under the stated convention, what happens to the overall median of an odd-sized list before forming the half-lists?
What you will learn
- Use all repeated observations when locating quartiles.
- Apply the stated median-of-halves convention for even and odd sample sizes.
- Interpret Q3 − Q1 as central spread and distinguish it from the whole range.
For 14, 7, 10, 3, 18, 9, 12, use the stated median-of-halves convention to find the median, lower quartile, upper quartile and IQR. Compare IQR with range. Use medians of the lower and upper halves, excluding the overall middle observation when the total count is odd.
Enter your own actual observations, positions, products, operations and guide endpoints. No count, total, role, result, quartile, graph height, unit or precision is filled for you. Recording is an attempt, not a correctness result.
Original supplied observations and tables
Only literal observations, frequencies, intervals and supplied cumulative guides are shown initially. The display does not sort observations, compute a total, average, quartile, rank or recovered value, or prefill a learner operation.
A five-number display appears only after you enter Q1, median and Q3 in the original common unit and explicitly record both extremes. Missing values are not derived and unit conversions are not silently plotted.
Blue denotes literal supplied guides, amber actual entered work, purple dirty retained work, and green a separately gated reference. Wrong and partial values are never repaired. The drawing bound is±1,000,000 coordinates, with at most512 scalar entries and128 cumulative points; these are tool bounds, not mathematical laws. Native clipping does not clamp values. No completed target is silently substituted.
The selected median-of-halves convention excludes the overall middle observation for odd lists. Grouped means and guide read-offs remain estimates. A zero-frequency plateau can give an interval rather than a unique reading. Fixed guide comparisons use the separate canonical700×500 policy with each source total; the tolerance is not a current-screen pixel or official marking tolerance. Text presence, operation consistency and numerical self-check do not approve reasoning, statistical claims, marks, mastery, teacher/specialist or formal assessment. No probability or new boxplot teaching credit is granted.
Order all observations and mark both halves and central positions under the stated convention
Observation positions start at 1. Enter the repeated observations and each selected position yourself.
No actual work is recorded for this action.
This is symbolic or one-dimensional work. These controls do not turn observation positions, row indices, extremes or interval boundaries into Cartesian points.
My quantities, conclusions and explanation
For a numerical list, use comma-separated values with no brackets or one balanced enclosing pair, or type none when you mean no mode. State the actual unit and exact, estimate or one-decimal-place precision yourself.
Retain the stated data, units, signs and precision. Numerical agreement is a self-check only.
Retain the stated data, units, signs and precision. Numerical agreement is a self-check only.
Retain the stated data, units, signs and precision. Numerical agreement is a self-check only.
Retain the stated data, units, signs and precision. Numerical agreement is a self-check only.
Retain the stated data, units, signs and precision. Numerical agreement is a self-check only.
A written attempt needs actual reasoning; text presence does not approve the argument.
A written attempt needs actual reasoning; text presence does not approve the argument.
These are original fixed cases for self-checking, not automatic marking, mastery or teacher/specialist/formal-assessment approval. The stated median-of-halves convention is a chosen convention. Grouped midpoint means and within-class CF readings remain estimates; a plateau can give a full interval. A physical instrument and calibrated graph are separate from the screen. No probability credit follows.
State which quartile convention is being used
Quartiles divide an ordered distribution into lower and upper parts, and the interquartile range is the upper quartile minus the lower quartile. The terms lower quartile and upper quartile are often written Q1 and Q3. They are values on the data scale, so subtracting them gives a spread in the original unit.
These discrete-data tasks explicitly use medians of halves: order all observations, split an even-sized list into equal lower and upper halves, and find each half’s median. For an odd-sized list, first locate the overall median and exclude that single middle observation from both halves. This convention is stated for these tasks; a task that omits a convention needs care before rejecting another defensible rule.
Use the actual half-lists
Write the lower and upper half-lists or identify their positions clearly. If a half contains an even number of observations, average that half’s two middle values. If it contains an odd number, use its single middle value. Keep repeated readings as separate observations, including readings that happen to equal the overall median.
A frequency table needs the same observation-level reasoning. Expand the frequencies when the list is short, or locate the necessary positions using cumulative counts. Taking quartiles of the printed row labels alone gives each distinct value equal weight and can produce a different answer from the quartiles of the data that were actually observed.
Subtract quartiles and interpret cautiously
After finding the quartiles, calculate IQR = Q3 − Q1. Keep signs and fractional values intact until the subtraction is complete; a negative lower quartile is a value, not a negative spread. Since Q3 is not below Q1 in an ordered distribution, the IQR cannot be negative.
IQR concerns the distance between the two quartile values, whereas range concerns the minimum and maximum. An IQR of zero can occur when the middle data cluster at one value even though the extremes differ. The source requires discrete IQR, but does not prescribe one positional algorithm here; the stated classroom convention does not certify an unseen mark scheme or a full assessment policy. Both half-lists must contain observations; an empty half has no defined median.
Sources for this lesson
- Pearson Mathematics A linear specification · Issue 2 · Foundation printed24/physical30; Higher printed39/physical45; inheritance printed27/physical33
- Pearson 2026/2027 entry manual · centre regions physical3; Mathematics A physical25
- Pearson second-edition modular book/Hub map · literal navigation only · physical9,10,11
Pause and explain
Using the stated convention on 1, 2, 4, 8, 11, what is the IQR?
Worked example
For 14, 7, 10, 3, 18, 9, 12, use the stated median-of-halves convention to find the median, lower quartile, upper quartile and IQR. Compare IQR with range. Use medians of the lower and upper halves, excluding the overall middle observation when the total count is odd.
Show the worked solution
- Order as 3, 7, 9, 10, 12, 14, 18. The median is 10; omit it from both halves under the stated odd-count convention.
- The lower half 3, 7, 9 has median Q1 = 7. The upper half 12, 14, 18 has median Q3 = 14.
- IQR = 14 − 7 = 7 units; range = 18 − 3 = 15 units. These measure different portions of the distribution.
Answer Under the stated median-of-halves convention: median = 10, Q1 = 7, Q3 = 14, IQR = 7 and range = 15 units. Retain every repeated observation and exclude only the overall middle position when the full count is odd.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →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.
For 5, 7, 9, 11, 13, 15, 17, 19, find median, Q1, Q3, IQR and range using median-of-halves. Show which two readings determine each quartile. Use medians of the lower and upper halves, excluding the overall middle observation when the total count is odd.
Give me a hint
Split the eight observations into two groups of four and find each group’s central pair.
Compare my reasoning
- Eight observations split into lower four 5, 7, 9, 11 and upper four 13, 15, 17, 19. Overall median = (11 + 13)/2 = 12.
- Q1 = (7 + 9)/2 = 8; Q3 = (15 + 17)/2 = 16.
- IQR = 16 − 8 = 8 and range = 19 − 5 = 14. Quartiles may lie between observed values.
Under the stated median-of-halves convention: median = 12, Q1 = 8, Q3 = 16, IQR = 8 and range = 14 units. Retain every repeated observation and exclude only the overall middle position when the full count is odd.
Look for these in your work
- State and apply the given convention.
- Retain repeated observations and locate the two halves.
- Subtract Q1 from Q3 with units.
For −9, −3, −1, 0, 2, 4, 4, 7, 11, calculate median, quartiles, IQR and range using the stated convention. Explain the subtraction when Q1 is negative. Use medians of the lower and upper halves, excluding the overall middle observation when the total count is odd.
Give me a hint
Exclude the single overall middle position, retain the other repeated values, and subtract the negative Q1 correctly.
Compare my reasoning
- The middle (fifth) value is 2; exclude it, leaving four values in each half.
- Q1 = (−3 + (−1))/2 = −2. Q3 = (4 + 7)/2 = 5.5.
- IQR = 5.5 − (−2) = 7.5 units, which is nonnegative; range = 11 − (−9) = 20.
Under the stated median-of-halves convention: median = 2, Q1 = -2, Q3 = 5.5, IQR = 7.5 and range = 20 units. Retain every repeated observation and exclude only the overall middle position when the full count is odd.
Look for these in your work
- State and apply the given convention.
- Retain repeated observations and locate the two halves.
- Subtract Q1 from Q3 with units.
A discrete frequency table gives values 0, 1, 2, 3, 4, 5 with frequencies 1, 2, 3, 3, 2, 1. Find the median, Q1, Q3, IQR and range using median-of-halves. Explain why taking the quartiles of the six row labels would be wrong. Use medians of the lower and upper halves, excluding the overall middle observation when the total count is odd.
Give me a hint
Reconstruct the twelve observations or their positions before dividing them into two half-lists.
Compare my reasoning
- Expand or locate all twelve observations: 0, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 5. Median = (2 + 3)/2 = 2.5.
- The lower six have central pair 1 and 2, so Q1 = 1.5. The upper six have central pair 3 and 4, so Q3 = 3.5.
- IQR = 2 and range = 5 units. The row labels alone discard the unequal repetitions and are not the data set.
Under the stated median-of-halves convention: median = 2.5, Q1 = 1.5, Q3 = 3.5, IQR = 2 and range = 5 units. Retain every repeated observation and exclude only the overall middle position when the full count is odd.
Look for these in your work
- State and apply the given convention.
- Retain repeated observations and locate the two halves.
- Subtract Q1 from Q3 with units.
Common mistakes
- Including an odd list’s overall median in both halves despite the stated exclusion convention.
- Finding quartiles of distinct row labels instead of repeated observations.
- Adding the quartiles or dropping a negative sign instead of calculating Q3 − Q1.
What can you explain now?
If IQR = 0, must every observation be equal?
Compare with the explanation
No. The two quartiles can coincide while lower and upper extremes differ.
IQR concerns the central spread between quartiles and does not by itself determine the minimum, maximum or entire distribution.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
State the adopted convention, explain how repeated frequencies determine positions, and distinguish median, Q1, Q3, IQR and range.
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.