
4-8 Standard deviation of a grouped (class) distribution
Point 8 of 12 — Unit 4: Statistics — Measures of DispersionQuestion bank: 0 questions (0 with pictures)
08 4-8 Standard deviation of a grouped (class) distribution
- Replace each class by its midpoint x = (lower + upper)/2 and treat the table as a frequency distribution with those midpoints.
- Then apply σ² = Σfx²/Σf − (Σfx/Σf)²; the answer is an estimate because the real values inside a class are unknown.
- Use class boundaries, not the printed limits, when computing midpoints of continuous classes such as 10–19, 20–29.
- Quartiles of grouped data are read from the cumulative-frequency curve or located in the class containing the required position.
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