Decode shifted and scaled totals without losing units
Recover original means and spreads from coded observations, separating changes in origin from changes in scale.
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.
- Expand a squared binomial and rearrange a linear equation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
If y = x − 100 and mean y = 2, what is mean x?
What you will learn
- Recover original moments from totals of x − a.
- Transform means and standard deviations under linear coding.
A shifted origin simplifies working
Let y = x − 100 for five measurements with Σy = 10 and Σy² = 30. Mean y = 2 and variance y = 30/5 − 2² = 2. Since x = y + 100, mean x = 102 and standard deviation x = √2, unchanged by the shift. If original totals are needed, Σx = Σy + 100n = 510 and Σx² = Σy² + 200Σy + 10000n = 52030.
A shift changes every observation and mean by the same amount, so deviations remain unchanged. Do not add 100 to a standard deviation or square Σy to replace Σy²; these totals represent different operations. Coding is useful when large values are clustered near a convenient origin, but the count and units must remain clear.
A scale factor changes spread by its magnitude
For z = (x − 50)/2 with n = 4, Σz = 4 and Σz² = 12, mean z = 1 and variance z = 2. Since x = 50 + 2z, original mean is 52 and original standard deviation is 2√2; original variance is 8. In general, x = a + bz gives mean x = a + b mean z and standard deviation x = |b| times standard deviation z.
Variance scales by b², while standard deviation scales by |b| so it never becomes negative. For w = 10 − 3x when x has mean 4 and standard deviation √2, mean w = −2 and standard deviation w = 3√2. A negative scale reverses ordering but does not create a negative spread. Translate coded totals back to a common coding before combining different groups.
Pause and explain
For x = 50 + 2z and standard deviation z = √2, what is standard deviation x?
Worked example
For n = 5, Σ(x − 100) = 10 and Σ(x − 100)² = 30, find original mean and descriptive standard deviation.
Show the worked solution
- Set y = x − 100, so mean y = 10/5 = 2.
- Variance y = 30/5 − 2² = 2, giving standard deviation √2.
- Add the origin to the mean only: mean x = 102 and standard deviation x = √2.
Answer Original mean 102 and standard deviation √2; Σx = 510 and Σx² = 52030.
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.
Five observations have Σ(x − 100) = 10 and Σ(x − 100)² = 30. Find mean, standard deviation and original first and second totals.
Give me a hint
Compute coded mean and variance first, then expand x = y + 100.
Compare my reasoning
- Mean y = 2 and variance y = 30/5 − 4 = 2.
- Mean x = 102 and standard deviation x = √2.
- Σx = 510; Σx² = 30 + 200(10) + 10000(5) = 52030.
Mean 102; standard deviation √2; Σx 510, Σx² 52030.
Look for these in your work
- Undo the shift only in the mean.
- Include the cross term in the squared total.
For z = (x − 50)/2, four observations have Σz = 4 and Σz² = 12. Find original mean, variance and standard deviation.
Give me a hint
Undo coding with x = 50 + 2z.
Compare my reasoning
- Mean z = 1 and variance z = 12/4 − 1² = 2.
- Mean x = 52 and variance x = 2²(2) = 8.
- Standard deviation x = 2√2; original totals are 208 and 10848.
Mean 52; variance 8; standard deviation 2√2; original totals 208 and 10848.
Look for these in your work
- Scale variance and standard deviation differently.
- Retain the original units after decoding.
Data x = 2, 4, 4, 6 have mean 4 and standard deviation √2. For w = 10 − 3x, find transformed values, mean and standard deviation.
Give me a hint
A negative scale reverses values, but spread uses its magnitude.
Compare my reasoning
- The transformed values are 4, −2, −2, −8.
- Their mean is 10 − 3(4) = −2.
- Their variance is 9(2) = 18 and standard deviation 3√2, which is non-negative.
Values 4, −2, −2, −8; mean −2; variance 18; standard deviation 3√2.
Look for these in your work
- Check the transformed observations directly.
- Use absolute scale for the standard deviation.
Common mistakes
- Σy² and (Σy)² are different totals.
- A shift changes mean but not spread.
- Use absolute scale for standard deviation and squared scale for variance.
What can you explain now?
If w = 10 − 3x and x has standard deviation √2, can w have standard deviation −3√2?
Compare with the explanation
No; its standard deviation is 3√2.
The negative coefficient reverses order, while the magnitude multiplies deviations.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Undo the coded origin in the mean, apply scale magnitude to standard deviation, and reconcile codes before adding 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.