In the formula 
for finding the mean of grouped data di’s are deviation from a of
We know that, di = xi - a
Where,
xi are data and “a” is the assumed mean
i.e. di are the deviations from of mid - points of the classes.
1
In the formula for finding the mean of grouped data di’s are deviation from a of
We know that, di = xi - a
Where,
xi are data and “a” is the assumed mean
i.e. di are the deviations from of mid - points of the classes.