Standard Deviation
A measure of dispersion representing the average distance of data values from the mean. Calculated as s = √(Σ(x - x̄)²/(n-1)) for a sample or σ = √(Σ(x - μ)²/n) for a population.
Real World
Ofqual uses the standard deviation of A-level grade distributions to monitor whether results are becoming more or less spread out year-on-year, which was especially scrutinised after the 2020 algorithm controversy.
Exam Focus
Use (n−1) for sample standard deviation — losing a mark for dividing by n instead of (n−1) is one of the most common errors.
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