Calculate descriptive standard deviation from raw and frequency data
Measure deviations about the mean, use weighted moments for repeated values and distinguish standard deviation from variance.
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
- Calculate mean and squared values.
- Expand and simplify basic algebraic expressions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For 2, 4, 4, 6, what is n?
What you will learn
- Calculate descriptive variance and standard deviation with divisor n.
- Use frequencies correctly in both first and second moments.
Square deviations before averaging
For 2, 4, 4, 6 minutes, mean is 4. Deviations −2, 0, 0, 2 sum to zero, so their signed average cannot measure spread. Squared deviations 4, 0, 0, 4 have mean 8/4 = 2. Descriptive variance is 2 minutes squared and standard deviation is √2 ≈ 1.414 minutes.
The equivalent moment formula is variance = Σx²/n − (Σx/n)². Here Σx = 16 and Σx² = 72, giving 72/4 − 4² = 2. The square root returns spread to the original units. These are descriptive summaries of the supplied values, so use divisor n; the n − 1 sample variance estimator belongs to a different inference task.
Frequency multiplies both moments
For values 1, 2, 4 with frequencies 2, 3, 1, there are n = 6 observations. Σfx = 2(1) + 3(2) + 1(4) = 12 and Σfx² = 2(1²) + 3(2²) + 1(4²) = 30. Mean is 2, variance 30/6 − 2² = 1, and standard deviation 1. Counting three distinct values as n would ignore repetitions.
Compute f × x², not (fx)². Variance is unchanged if the same constant is added to every observation; deviations from the new mean are identical. Keep enough working digits because subtracting similar large moments can magnify rounding error. A negative computed variance signals incompatible data or arithmetic/rounding trouble, not a negative standard deviation to accept.
Pause and explain
For variance 2 minutes squared, what is standard deviation?
Worked example
Find descriptive mean, variance and standard deviation of 2, 4, 4, 6 minutes.
Show the worked solution
- n = 4, Σx = 16 and Σx² = 72, so mean = 4 minutes.
- Variance = 72/4 − 4² = 2 minutes squared, also the average squared deviation.
- Standard deviation = √2 ≈ 1.414 minutes.
Answer Mean 4 minutes; variance 2 minutes squared; standard deviation √2 ≈ 1.414 minutes.
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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.
Find the descriptive mean, variance and standard deviation of 2, 4, 4, 6 minutes.
Give me a hint
Use total 16 and squared total 72 with n = 4.
Compare my reasoning
- Mean = 16/4 = 4 minutes.
- Variance = 72/4 − 4² = 2 minutes squared.
- Standard deviation = √2 ≈ 1.414 minutes.
Mean 4; variance 2; standard deviation √2 ≈ 1.414 minutes.
Look for these in your work
- Use divisor 4 and retain the repeated observation.
- Take the square root for standard deviation.
Values 1, 2, 4 marks have frequencies 2, 3, 1. Find n, mean and descriptive standard deviation.
Give me a hint
Sum f, fx and fx² separately.
Compare my reasoning
- n = 6; Σfx = 12 and Σfx² = 30.
- Mean = 12/6 = 2 marks and variance = 30/6 − 2² = 1 mark squared.
- Standard deviation = 1 mark.
n = 6; mean 2 marks; variance 1; standard deviation 1 mark.
Look for these in your work
- Count all frequency repetitions.
- Use f x² rather than (fx)².
Measurements 12, 14, 14, 16 minutes come from adding 10 to 2, 4, 4, 6. Calculate their moments and show what changes and what stays fixed.
Give me a hint
A shift changes centre but not deviations about the mean.
Compare my reasoning
- Σx = 56 and Σx² = 792; mean = 14 minutes.
- Variance = 792/4 − 14² = 2 minutes squared.
- Standard deviation remains √2 minutes; the mean rose by 10 minutes.
Mean 14 minutes; variance 2 and standard deviation √2 minutes, unchanged by the shift.
Look for these in your work
- Verify the shifted moments independently.
- Explain unchanged deviations rather than averaging standard deviations.
Common mistakes
- Use n for these descriptive summaries.
- Weight x² by f rather than squaring fx.
- Standard deviation has original units; variance has squared units.
What can you explain now?
Why is the average signed deviation from the mean not a useful spread measure?
Compare with the explanation
It is zero because positive and negative deviations cancel.
Squared deviations retain their magnitude before averaging.
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Make it stick
Count observations, calculate first and second weighted moments, subtract mean squared, then take the non-negative square root.
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.