Question:
In the formula $\bar{x}=a+\frac{\Sigma f_{i} d_{i}}{\Sigma f_{i}}$, for finding the mean of grouped data $d_{i}$ 's are deviation from $a$ of
(a) lower limits of the classes
(b) upper limits of the classes
(c) mid-points of the classes
(d) frequencies of the class marks
Solution:
(c) We know that, $d_{i}=x_{i}-\mathrm{a}$
i.e., $d_{i}$ 's are the deviation from a of mid-points of the classes.