Estimate mean and standard deviation from grouped intervals
Use each class midpoint for moment estimates while explaining why precise arithmetic cannot reconstruct the original measurements.
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
- Use frequency-weighted first and second moments.
- Find numerical interval midpoints.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What representative midpoint is used for [20,40)?
What you will learn
- Estimate grouped mean and standard deviation using midpoint moments.
- State the information lost by grouping and keep estimates labelled.
Use a representative midpoint in each class
For classes [10,20), [20,30), [30,40) marks with frequencies 2, 6, 2, use midpoints 15, 25, 35. n = 10, Σfm = 250 and Σfm² = 6650. Estimated mean is 25 and estimated variance is 665 − 625 = 40; estimated standard deviation is 2√10 ≈ 6.325 marks. The formula weights midpoint squares, not upper-boundary squares.
Replacing each unknown value by its class midpoint is a modelling assumption. The actual observations can lie elsewhere within their intervals, so neither the raw mean nor standard deviation is generally exact. Choosing narrower classes can reduce lost location information, but does not prove every estimate is closer in every dataset. Keep the estimate label even when a midpoint total happens to be an integer.
Unequal widths do not change frequency weighting
For times [0,10), [10,20), [20,40), [40,50) with frequencies 4, 12, 8, 6, use midpoints 5, 15, 30, 45. Then n = 30, Σfm = 710 and Σfm² = 22150. Estimated mean is 71/3 ≈ 23.67 minutes; estimated variance is 1604/9 and standard deviation 2√401/3 ≈ 13.35 minutes.
Frequencies weight the moments. Frequency densities are histogram heights and must first be converted to counts using class widths if only a density histogram is supplied. Averaging midpoints without their weights treats each class as equally frequent, which is a different dataset. Retain unrounded mean and second moment until the final square root.
Pause and explain
For n = 10, Σfm = 250 and Σfm² = 6650, the estimated variance is…
Worked example
Estimate mean and standard deviation for [10,20), [20,30), [30,40) marks with frequencies 2, 6, 2.
Show the worked solution
- Midpoints 15, 25, 35 give n = 10, Σfm = 250 and Σfm² = 6650.
- Estimated mean = 25 and variance = 6650/10 − 25² = 40.
- Estimated standard deviation = √40 = 2√10 ≈ 6.325 marks; individual within-class positions remain unknown.
Answer Estimated mean 25 marks; variance 40; standard deviation 2√10 ≈ 6.325 marks.
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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.
Estimate mean and standard deviation for classes [10,20), [20,30), [30,40) with counts 2, 6, 2 marks.
Give me a hint
Use midpoint totals 250 and 6650 for ten observations.
Compare my reasoning
- Midpoints 15, 25, 35 give estimated mean 250/10 = 25 marks.
- Estimated variance = 6650/10 − 25² = 40.
- Estimated standard deviation = 2√10 ≈ 6.325 marks.
Estimated mean 25 and standard deviation 2√10 ≈ 6.325 marks.
Look for these in your work
- Weight midpoint squares by frequencies.
- Retain estimate language.
For time classes [0,10), [10,20), [20,40), [40,50) and counts 4, 12, 8, 6, calculate midpoint totals and estimate mean and standard deviation.
Give me a hint
The unequal-width class [20,40) has midpoint 30 and count 8.
Compare my reasoning
- Midpoints 5, 15, 30, 45 give Σfm = 710, Σfm² = 22150, n = 30.
- Mean estimate = 71/3; variance estimate = 22150/30 − (71/3)² = 1604/9.
- Standard-deviation estimate = 2√401/3 ≈ 13.35 minutes.
Totals 710 and 22150; estimated mean 71/3 ≈ 23.67 and standard deviation 2√401/3 ≈ 13.35 minutes.
Look for these in your work
- Use frequency 8 rather than density 0.4 as the third weight.
- Keep unrounded moments in the variance calculation.
Two actual datasets each place two observations in [10,20) and two in [20,30). Dataset A is 10, 10, 20, 20; B is 19, 19, 29, 29. Compare their raw means with the common midpoint mean estimate.
Give me a hint
Both grouped tables match, but their observations sit at different class locations.
Compare my reasoning
- Both midpoint tables use 15 twice and 25 twice, giving estimated mean 20.
- A has raw mean 15 while B has raw mean 24.
- The common grouped estimate does not determine either exact raw mean.
Common midpoint mean estimate 20; actual means A 15 and B 24.
Look for these in your work
- Verify both datasets satisfy the same interval counts.
- Use the example to explain lost within-class information.
Common mistakes
- Use midpoints and frequencies, not class endpoints or density weights.
- Label mean and standard deviation as estimates.
- Keep working precision before subtracting moments.
What can you explain now?
Does an estimated grouped mean of exactly 25 prove the raw mean was 25?
Compare with the explanation
No; individual positions inside each interval are unknown.
Exact midpoint arithmetic does not remove grouping uncertainty.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Replace unknown within-class values by midpoints, weight by counts, calculate moments and preserve the estimate label.
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.