Assumed mean method
- CBSE · NCERT Class 10, Ch 13 Statistics
The assumed mean method finds the mean of grouped data by picking one class mark as a starting guess and adjusting it.
Large class marks make big products. To keep the numbers small, pick one class mark near the middle and call it a. Then find how far each class mark is from a. This gap is d. The mean is a + (sum of f × d) ÷ (sum of f). The step-deviation method goes one step further. It divides every d by the class size h.
A small example
Classes 0-10, 10-20 and 20-30 have frequencies 2, 3 and 5. The class marks are 5, 15 and 25. Take a = 15. The gaps d are −10, 0 and 10. The products f × d are −20, 0 and 50, which add to 30. So the mean is 15 + 30 ÷ 10 = 18. The direct method gives the same answer.
Where marks go
Question 30(A) of CBSE's 2026-27 Maths Basic sample paper gives the mean, 48, and asks for two missing frequencies. The 2026-27 Maths Basic marking scheme works it with a = 45 and h = 10. The table earns 1 mark.
Four more steps earn half a mark each. They are: stating a and h, the equation from the total frequency, the equation for the mean, and the two missing values. Only the last half mark depends on the final answers. See how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
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