In such cases the missing frequencies are represented by f₁ or f₂ as the case may be and then the formula for the calculation of median is used. In this process we get an equation which gives us the missing values. The following example would clarify the procedure.

Example : In the frequency distribution of 100 families given below, the number of families corresponding to expenditure groups 20—40 and 60—80 are missing from the table. However, the median is known to be 50. Find the missing frequencies. 

 

Expenditure	0—20	20—40	40—60	60—80	80—100
No. of families	14	?	27	?	15

Solution: Let the missing frequency of the group 20—40 be f₁ and the missing frequency of 60—80 group be f₂.

        Now              f₂ = 100 (Total frequency) — (14+f₁+27+15)                       

30—40	?
40—50	65
50—60	?
60—70	25
70—80	18
	229