Question

what assumption we make while counting the median of a grouped data

Answer 1

If observations of a variable are ordered by value, the median value corresponds to the middle observation in that ordered list. The median value corresponds to a cumulative percentage of 50% (i.e., 50% of the values are below the median and 50% of the values are above the median). The position of the median is  {(n + 1) ÷ 2}th value, where n is the number of values in a set of data.  In order to calculate the median, the data must first be ranked (sorted in ascending order). The median is the number in the middle.