Find raw-data centre, quartiles and spread
Order measurements, select the right positions and distinguish a typical value from overall and middle-half variation.
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 data including repeated values.
- Calculate sums and averages.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For eight ordered values, which positions determine the median?
What you will learn
- Calculate mean, median, mode, range and IQR for raw data.
- Explain sensitivity to extremes and the stated raw quartile convention.
Position and frequency answer different questions
For ordered values 2, 3, 3, 4, 8, 9, 10, 11 minutes, the mean is 50/8 = 6.25 and median is (4 + 8)/2 = 6 minutes. The mode is 3 because it appears most often; it need not equal either centre measure. The range is 11 − 2 = 9 minutes.
A median uses ordered positions, not half of the sum. With an even count average the two middle values; with an odd count use the single middle value. A dataset can have several modes, or no uniquely most frequent value. Do not automatically choose the smallest value when frequencies tie. Report units alongside every measure.
Specify the raw quartile convention
In this pathway’s raw-data questions, take medians of the lower and upper ordered halves, excluding the overall median for an odd count. The lower half 2, 3, 3, 4 gives Q1 = 3; the upper half 8, 9, 10, 11 gives Q3 = 9.5. The IQR is 9.5 − 3 = 6.5 minutes. Follow a convention provided in an exam question rather than silently switching definitions.
The mean uses every value and can move strongly when an extreme value changes. The median and IQR focus on positions and can remain unchanged. Replacing 11 with 101 in this eight-value dataset raises the mean to 140/8 = 17.5 and the range to 99, while median stays 6; Q3 remains 9.5 under the stated convention. A robust summary is useful for typical experience, but extreme values can still be substantively important.
Pause and explain
If Q1 = 3 and Q3 = 9.5 minutes, what is the IQR?
Worked example
For 2, 3, 3, 4, 8, 9, 10, 11 minutes, calculate mean, median, mode, range and IQR using medians of the two halves.
Show the worked solution
- Sum = 50, so mean = 6.25; middle pair 4 and 8 gives median 6; the mode is 3.
- Range = 11 − 2 = 9; lower-half median is Q1 = 3 and upper-half median Q3 = 9.5.
- IQR = 9.5 − 3 = 6.5 minutes.
Answer Mean 6.25; median 6; mode 3; range 9; Q1 3, Q3 9.5, IQR 6.5 minutes.
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.
Find mean, median and mode of 2, 3, 3, 4, 8, 9, 10, 11 minutes.
Give me a hint
Use the sum for the mean, middle pair for the median and greatest frequency for mode.
Compare my reasoning
- The sum is 50 and count 8, giving mean 6.25 minutes.
- The ordered middle values 4 and 8 give median 6 minutes.
- The value 3 appears twice and all others once, so mode is 3 minutes.
Mean 6.25, median 6 and mode 3 minutes.
Look for these in your work
- Do not confuse the middle position with the mean.
- Use repetition to identify the mode.
For ordered values 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 marks, exclude the overall median before taking medians of the two halves. Find Q1, median, Q3, IQR and range.
Give me a hint
There are five values below 6 and five above it.
Compare my reasoning
- The overall median is the sixth value, 6 marks.
- Lower half 1–5 has median 3; upper half 7–11 has median 9.
- IQR = 9 − 3 = 6 and range = 11 − 1 = 10 marks.
Q1 3, median 6, Q3 9; IQR 6, range 10 marks.
Look for these in your work
- Exclude the overall median under the stated convention.
- Keep middle-half and whole-range spreads separate.
Replace 11 with 101 in 2, 3, 3, 4, 8, 9, 10, 11 minutes. Compare the new mean, median, range and IQR with the originals using the same quartile rule.
Give me a hint
The sum rises by 90 but the middle positions stay fixed.
Compare my reasoning
- The new sum is 140, so mean = 17.5 minutes; the middle pair still gives median 6.
- The new range is 101 − 2 = 99 minutes.
- Q1 remains 3 and Q3 remains 9.5; IQR remains 6.5 minutes.
Mean 17.5, median 6, range 99 and IQR 6.5 minutes; the median and IQR are unchanged.
Look for these in your work
- Recalculate the sum and extremes.
- Explain positional stability under this specific change rather than claiming universal immunity.
Common mistakes
- Order values before finding positional summaries.
- IQR is Q3 − Q1, not maximum minus minimum.
- State the quartile convention and keep units.
What can you explain now?
If only the largest value 11 becomes 101 here, must the median increase?
Compare with the explanation
No; its middle positions still contain 4 and 8.
The median stays 6 even though the sum, mean and range change.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Use sum for mean, positions for median and quartiles, frequencies for mode, and the correct pair of endpoints for each spread.
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.