Choose a representation that answers a statistical question
Match the variable and purpose to a diagram, explaining which information it preserves and what it hides.
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
- Distinguish a category from a numerical measurement.
- Read a frequency table and identify its units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which variable is a numerical measurement suitable for grouping into intervals?
What you will learn
- Choose representations for raw numerical and grouped data.
- Explain a specific advantage and limitation in context.
Choose for the question and the data
For eight measured journey times, a stem-and-leaf diagram can retain every recorded time and show their ordering. A histogram suits many numerical measurements grouped into intervals, especially unequal-width classes: its area, rather than its height alone, represents frequency. A bar chart for named categories keeps the categories separate; a histogram uses a numerical scale and contiguous class boundaries.
A cumulative frequency graph answers questions about cutoffs, such as an estimated time below which 80% of journeys lie. A box-and-whisker plot compares median and quartiles on a common scale, but does not retain every individual value. A back-to-back stem-and-leaf diagram compares two modest raw datasets while preserving their values. State the variable and units before choosing a representation.
Explain what a summary cannot establish
A box plot may show the same five-number summary for different raw datasets. It cannot usually give an exact mean, mode or sample size. A grouped histogram hides the locations of values inside each class. A stem-and-leaf plot preserves recorded precision, but becomes crowded for thousands of measurements or awkwardly spread values.
For delivery times, a lower median and smaller interquartile range may describe faster and more consistent typical deliveries in the recorded data. Neither a chart nor those summaries alone proves the delivery service caused the difference, represents all future customers or is better on every criterion. Use comparable units, observation conditions and honest axis scales.
Pause and explain
Which representation retains all eight recorded numerical values with a suitable key?
Worked example
You have 12 raw journey times and want readers to recover the original values; later you want to estimate the 90th percentile from 300 grouped times. Choose a representation for each purpose and give a limitation.
Show the worked solution
- Use an ordered stem-and-leaf diagram with a time-unit key for the 12 raw values.
- Use a cumulative frequency graph for the grouped percentile question, plotting cumulative counts at upper boundaries.
- The stem diagram can become crowded; the grouped percentile is an estimate because locations within classes are unknown.
Answer Stem-and-leaf for recoverable raw values; cumulative frequency for a grouped percentile estimate.
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.
Choose a representation for 10 raw test scores when the reader must recover every score. Give a benefit and a limitation.
Give me a hint
Ask which diagram retains one visible entry per score.
Compare my reasoning
- Choose an ordered stem-and-leaf diagram and provide a score key.
- Each repeated leaf records a separate score and the ordering shows concentration.
- It retains all recorded scores, but becomes crowded for a very large dataset.
Ordered stem-and-leaf; recoverable scores, limited readability for large datasets.
Look for these in your work
- Tie the benefit to recovering values.
- State a genuine limitation without claiming values were grouped.
Two classes have 200 grouped completion times each. Choose a display to compare typical time and middle-half spread; name information that the display cannot recover.
Give me a hint
A five-number summary puts two distributions on a shared measurement axis.
Compare my reasoning
- Use box-and-whisker plots on the same time scale, deriving estimated quartiles from grouped data.
- Compare medians for typical time and IQRs for the middle-half spread.
- The box plots do not recover every time or an exact mean; grouped quartiles remain estimates.
Common-scale box plots; compare median and IQR, with grouped estimates and no exact raw-value recovery.
Look for these in your work
- Use the same time units and scale.
- Keep grouping and summary limitations explicit.
A report uses separated bars labelled 0–5, 5–15 and 15–35 minutes, with heights equal to frequencies 10, 20 and 20. Explain how to correct it for a continuous-time histogram.
Give me a hint
Different class widths require different frequency densities.
Compare my reasoning
- The widths are 5, 10 and 20 minutes.
- Density heights are 10/5 = 2, 20/10 = 2 and 20/20 = 1 per minute.
- Draw adjoining rectangles on a continuous time scale; their areas are proportional to 10, 20 and 20.
Use adjoining widths 5, 10, 20 and density heights 2, 2, 1; frequency is represented by area.
Look for these in your work
- Calculate heights from frequency divided by width.
- Explain why using frequency height alone distorts unequal classes.
Common mistakes
- Choose a diagram for a stated purpose, not just its appearance.
- Grouped data do not reveal individual observations.
- A descriptive comparison is not a causal conclusion.
What can you explain now?
Can two box plots with identical five-number summaries have different means?
Compare with the explanation
Yes; observations between the five summary points can differ.
A five-number summary does not fix the sum of all observations.
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
Name the question, choose a representation, and state one piece of information it keeps and one it loses.
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.