Build ordered stem-and-leaf diagrams with an unambiguous key
Keep repeated measurements, empty stems and recorded precision visible instead of turning a display into a list of different values.
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
- Order numerical values including repeats.
- Understand place value and measurement units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many leaves are needed for 12, 14, 14, 18?
What you will learn
- Construct and decode ordered stem-and-leaf diagrams.
- Use decimal and split-stem keys without losing observations.
One leaf records one observation
For times 12, 14, 14, 18, 21, 23, 26 and 29 minutes, stem 1 has leaves 2 4 4 8 and stem 2 has leaves 1 3 6 9. The key 1 | 2 means 12 minutes. Keep both 4 leaves: two journeys took 14 minutes. Leaves on the right of a stem are written in increasing order.
Count the leaves to check that all eight observations are present. Include empty intermediate stems when necessary so a gap in the measurement range remains visible. For times 12, 14, 31, a stem 2 with no leaves makes the gap clear; skipping its row can make the scale harder to read. A diagram key must identify the numeric value and units, not just say that the leaves are ordered.
Precision and splitting change the key
For 2.1, 2.4, 2.4 and 3.0 seconds, use stems 2 and 3 with key 2 | 1 = 2.1 seconds. A leaf 0 in stem 3 represents 3.0, not 30. The same characters can encode different scales, so the key matters. Do not invent extra decimal precision beyond the measurements.
If a row is crowded, split each stem into two rows for leaves 0–4 and 5–9. For 20, 22, 25, 27 and 29, the lower split of stem 2 has 0 2 and the upper split has 5 7 9. Label the split convention. Repeated stems without an explanation can be misread as duplicate measurements or a changed place value.
Pause and explain
Under key 2 | 1 = 2.1 seconds, what is 3 | 0?
Worked example
Draw an ordered stem-and-leaf diagram for 12, 14, 14, 18, 21, 23, 26, 29 minutes.
Show the worked solution
- Use tens as stems 1 and 2 and units as leaves.
- Write 1 | 2 4 4 8 and 2 | 1 3 6 9; there are eight leaves including the repeat.
- Add key 1 | 2 = 12 minutes.
Answer 1 | 2 4 4 8; 2 | 1 3 6 9; key 1 | 2 = 12 minutes.
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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.
Construct an ordered diagram for 12, 14, 14, 18, 21, 23, 26, 29 minutes and check its count.
Give me a hint
Use tens stems and preserve the duplicated 14.
Compare my reasoning
- Stem 1 has 2 4 4 8 and stem 2 has 1 3 6 9.
- The leaf count is 4 + 4 = 8, matching the raw dataset.
- Key 1 | 2 = 12 minutes makes the precision and unit explicit.
1 | 2 4 4 8; 2 | 1 3 6 9; 8 observations; key 1 | 2 = 12 minutes.
Look for these in your work
- Keep both 14 observations.
- Provide an interpretable key and count check.
Display 2.1, 2.4, 2.4, 3.0 seconds using whole-number stems. Recover the largest value from your diagram.
Give me a hint
The leaves represent tenths rather than units.
Compare my reasoning
- Write 2 | 1 4 4 and 3 | 0.
- Use key 2 | 1 = 2.1 seconds; four leaves represent four measurements.
- The largest entry 3 | 0 is 3.0 seconds.
2 | 1 4 4; 3 | 0; key 2 | 1 = 2.1 seconds; largest 3.0 seconds.
Look for these in your work
- Preserve the repeated 2.4.
- Decode the zero leaf using the supplied precision.
Split stem 2 into leaves 0–4 and 5–9 for scores 20, 22, 25, 27, 29. Explain how to label the two rows.
Give me a hint
State which half of the leaf digits belongs to each repeated stem.
Compare my reasoning
- The 0–4 row is 2 | 0 2.
- The 5–9 row is 2 | 5 7 9, giving five leaves in total.
- Label both row ranges and key 2 | 5 = 25 marks so the repeated stem is intentional.
Stem 2 (0–4): 0 2; stem 2 (5–9): 5 7 9; key 2 | 5 = 25 marks.
Look for these in your work
- Partition the leaf digits without overlap.
- Explain the repeated stem and measurement units.
Common mistakes
- Keep repeated leaves as repeated observations.
- Put a value and units in the key.
- Explain split stems and retain relevant empty rows.
What can you explain now?
What does 3 | 0 mean when the key is 2 | 1 = 2.1 seconds?
Compare with the explanation
3.0 seconds.
The leaf is the tenths digit under the supplied key.
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Make it stick
Order one leaf per observation and check the count, key, precision and any split or empty stems.
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.