When the mid-values of class-intervals are given the class-intervals are found by first finding out the difference between two mid-values and then subtracting half of it from each mid-value to find the lower limit and adding it to each mid-value to find the upper limit of the class-intervals.

Example: Compute median from the following data :  

Mid-values:	115	125	135	145	155	165	175	185	195
Frequency:	6	25	48	72	116	60	38	22	3
Cumulative frequency	6	31	79	151	267	327	365	387	390

Solution

Here we are given the mid-values of the class-intervals of a continuous frequency distribution. The difference between two mid-values is 10,so the class interval =10.

For mid value 115 the class will be 110-120

Mid value 125: class 120-130

Total N=390 ,so median will be 390/2 =195^th value which comes in class with mid value 155 ( in class 150-160)

L1=150

L2=160

M=390

C=151

F=116

M= 150 + = 170.6