Correct totals and infer missing observations consistently
Repair both moments when a value is corrected and use independent totals to check an inferred observation.
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 descriptive moment formulas and combined totals.
- Solve simple numerical equations for an unknown value.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Correcting one existing value from 12 to 8 changes the observation count how?
What you will learn
- Update first and second totals for a corrected or missing observation.
- Check count, moment consistency and rounding before accepting a result.
A correction changes the squared total separately
Five observations were summarised by Σx = 40 and Σx² = 360, but a recorded 12 should have been 8. Keep n = 5 and correct the sum to 40 − 12 + 8 = 36. Correct the squared sum to 360 − 12² + 8² = 280, not by subtracting (12 − 8)². The corrected mean is 7.2, variance 280/5 − 7.2² = 4.16 and SD = 2√26/5 ≈ 2.040.
If an extra observation is added, count increases as well as both totals; if an observation is removed, all three change. For 2, 4, 4, 6, adding 9 gives n = 5, sum 25 and squared sum 153. The new mean is 5, variance 153/5 − 25 = 28/5 and SD √(28/5). A correction in place is different from adding a new measurement.
Use the second total as an independent check
Six observations have sum 30 and squared sum 190. Five known values 2, 3, 4, 5, 6 have sum 20 and squared sum 90, so the missing value from the first total is 10. Its square is 100, agreeing with 190 − 90. The full mean is 5 and descriptive variance 190/6 − 25 = 20/3. If the supplied squared total were 180 instead, the inferred 10 would not agree; do not force incompatible exact data into one answer.
Moment formulas require Σx² ≥ (Σx)²/n, because descriptive variance cannot be negative. Rounded published means or SDs can introduce small inconsistencies; use exact supplied totals when available and explain rounding limits. Do not clamp an appreciably negative variance to zero or claim a unique missing value if the supplied information does not determine it.
Pause and explain
A squared sum 360 includes a wrong 12 that should be 8. The corrected squared sum is…
Worked example
For n = 5, sum 40 and squared sum 360, correct one observation from 12 to 8 and find the new mean and SD.
Show the worked solution
- The corrected sum is 36; the corrected squared sum is 360 − 144 + 64 = 280.
- Mean = 36/5 = 7.2; variance = 280/5 − (36/5)² = 104/25.
- SD = √104/5 = 2√26/5 ≈ 2.040; n remains 5.
Answer Corrected mean 7.2 and SD 2√26/5 ≈ 2.040; totals 36 and 280.
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.
Correct n = 5, Σx = 40, Σx² = 360 when one recorded 12 should be 8. Find mean and descriptive SD.
Give me a hint
Keep count fixed, but repair each moment with its own operation.
Compare my reasoning
- Corrected totals are 36 and 280, with n still 5.
- Mean = 7.2 and variance = 280/5 − 7.2² = 104/25 = 4.16.
- SD = 2√26/5 ≈ 2.040.
Totals 36 and 280; mean 7.2; variance 4.16; SD 2√26/5 ≈ 2.040.
Look for these in your work
- Replace both the value and its square.
- Keep the count unchanged and retain working precision.
Add observation 9 to raw data 2, 4, 4, 6. Find the new count, totals, mean and descriptive SD.
Give me a hint
Adding a measurement changes n as well as both totals.
Compare my reasoning
- Old totals 16 and 72 become 25 and 153, with n = 5.
- Mean = 25/5 = 5 and variance = 153/5 − 25 = 28/5.
- SD = √(28/5) ≈ 2.366.
n 5; totals 25 and 153; mean 5; variance 28/5; SD √(28/5) ≈ 2.366.
Look for these in your work
- Increase the count by one.
- Add 9², not just 9, to the second total.
Six observations have exact sum 30 and squared sum 190. Five known values are 2, 3, 4, 5, 6. Infer the sixth, find variance and explain what would change if the squared sum were instead 180.
Give me a hint
Use the first total, then verify its square with the independent second total.
Compare my reasoning
- Known sum 20 leaves missing x = 10; known squared sum 90 leaves 100, agreeing with 10².
- Mean = 5 and variance = 190/6 − 25 = 20/3.
- A squared total 180 would leave 90 instead of 100 and conflict with the exact first total; the data would be inconsistent.
Missing value 10; variance 20/3; squared total 180 would be inconsistent with the exact sum.
Look for these in your work
- Check the inferred observation against both totals.
- Report incompatible exact data rather than adjusting variance arbitrarily.
Common mistakes
- Correct x² by removing the old square and adding the new square.
- Distinguish replacing an observation from adding one.
- Check a missing value against both exact totals.
What can you explain now?
If exact totals imply a negative descriptive variance, should you take its absolute value?
Compare with the explanation
No; check the supplied data and arithmetic for inconsistency.
A negative variance is impossible for real observations; changing its sign hides the problem.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Repair count, sum and squared sum according to the event, then check non-negative variance and agreement of independent totals.
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.