Data summaries
Choose an average that fits the data and interpret it with an appropriate measure of spread.
Exam scope and assessment notes
Probability & Statistics 1 (Paper 5) · Mathematics 9709, 2026–2027. This overview practises raw and discrete-frequency summaries and interpreting basic box plots; it does not cover all of section 5.1, standard deviation or grouped/coded data. Written solutions use self-check criteria, not an official mark scheme. Specialist accuracy review has not been completed. The diagram uses separate illustrative data, not the values in the practice tasks. For quartiles in these tasks, exclude the overall median from an odd-sized list, then take the median of each half. Box-plot whiskers here extend to the minimum and maximum; no outlier-exclusion rule is applied.
Before you begin
- Order numerical data and distinguish a value from its position.
- Multiply values by frequencies and interpret units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Six recorded times are 18, 4, 11, 7, 15, 9 minutes. What is their median?
What you will learn
- Find the median of ordered data.
- Explain why a mean may be affected by outliers.
Order before selecting a position
The median divides ordered data into two halves. For an odd number n of values, it is at position (n+1)/2. For an even number, average the two central values. A position is an index into the sorted list, not the numerical answer.
The mean uses every value and can move strongly when an extreme observation is added. The median depends on order and is often more resistant to an outlier.
A centre does not describe everything
Two data sets can have the same mean but very different variability. The range is maximum minus minimum; other measures describe spread around the centre. When comparing distributions, discuss both typical value and spread, and use the context's units.
For a frequency table, treat frequencies as repeated observations. The total number of observations is the sum of frequencies, not the number of rows.
Sources for this lesson
Pause and explain
Delivery-time samples C and D have (median, IQR, range) of (8, 2, 30) and (10, 5, 12) minutes respectively. Which comparison is justified?
Worked example
Find the median of 8, 3, 15, 9, 7, 10.
Show the worked solution
- Sort the data: 3,7,8,9,10,15.
- There are six values; select positions 3 and 4.
- Average 8 and 9.
Answer 8.5
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.
A discrete table records waiting times of 4, 7 and 12 minutes with frequencies 3, 4 and 1 respectively. Find the number of observations, mean, median and range. Explain why averaging the three row values is inappropriate.
Give me a hint
Use frequencies as repeats. Locate the central positions using cumulative counts.
Compare my reasoning
- There are 3 + 4 + 1 = 8 observations. The total waiting time is 4×3 + 7×4 + 12×1 = 52 minutes.
- The mean is 52/8 = 6.5 minutes. The unweighted row average 23/3 would give each distinct time equal weight despite their different frequencies.
- The ordered list is 4, 4, 4, 7, 7, 7, 7, 12. Positions 4 and 5 are both 7, so the median is 7 minutes. The range is 12 − 4 = 8 minutes.
n = 8; mean 6.5 minutes; median 7 minutes; range 8 minutes.
Look for these in your work
- Use the total frequency as the number of observations.
- Calculate a frequency-weighted sum before dividing.
- Locate both central observations and retain time units.
Two seven-journey samples have times A: 5, 6, 7, 7, 7, 8, 44 and B: 4, 6, 8, 9, 10, 12, 14 minutes. Calculate each mean, median, IQR and range. Compare typical time and spread, and explain why the means and medians give different rankings. Use the stated quartile rule.
Give me a hint
Report the middle-half spread separately from the full range. Keep the large delay in the discussion.
Compare my reasoning
- A has total 84, mean 12 and median 7 minutes. Excluding the median, the halves are 5, 6, 7 and 7, 8, 44: Q1 = 6, Q3 = 8, IQR = 2 minutes. Its range is 44 − 5 = 39 minutes.
- B has total 63, mean 9 and median 9 minutes. Its half-list medians are Q1 = 6 and Q3 = 12, giving IQR = 6 minutes. Its range is 14 − 4 = 10 minutes.
- A has a lower median and a smaller middle-half spread, but a larger range. Its 44-minute delay lifts the mean above B’s. Median and IQR describe the typical time and middle spread without hiding that operationally important delay. Neither sample proves one service is always faster or establishes its long-run performance.
A: mean 12, median 7, IQR 2, range 39 minutes. B: mean 9, median 9, IQR 6, range 10 minutes.
Look for these in your work
- Calculate all four summaries using one explicit quartile convention.
- Compare centre and both measures of spread in context.
- Explain the effect of 44 without deleting it or generalising beyond the samples.
A learner says that identical basic box plots guarantee identical means. Test the claim using scores P: 2, 4, 5, 6, 7, 8, 12 and Q: 2, 4, 6, 6, 8, 8, 12 marks. Calculate both five-number summaries under the stated quartile rule, then both means. Identify information a box plot does not retain.
Give me a hint
A five-number summary records selected positions, not the sum of every observation.
Compare my reasoning
- Each sorted list has seven values and median 6. Excluding the median, both have Q1 = 4 and Q3 = 8. Their minimum and maximum are 2 and 12, so both summaries are (2, 4, 6, 8, 12) marks.
- P totals 44 marks and Q totals 46 marks. Their means are 44/7 and 46/7 marks, approximately 6.29 and 6.57. They are unequal despite identical five-number summaries.
- A basic box plot does not retain every internal observation or its total. These explicit datasets disprove the claim; a mean cannot in general be recovered from the five-number summary alone.
Same summary (2, 4, 6, 8, 12); different means 44/7 and 46/7 marks.
Look for these in your work
- Check both medians and both half-list medians.
- Compute each mean from all seven scores.
- Use the unequal means as a counterexample, not a claim that box plots are useless.
Common mistakes
- Do not find a median from an unsorted list.
- The median of an even-sized set need not be one of its observations.
What can you explain now?
Which changes more when 100 replaces 10 in 2,4,6,8,10: mean or median?
Compare with the explanation
The mean
The median stays 6. The mean rises from 6 to 24.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain why median position is not median value, why a frequency-table mean needs weights, and how a smaller IQR can coexist with a larger range. State one feature that a basic box plot cannot determine.
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.