(Q11) The following data gives the information on the observed lifetimes (in hours) of 225 electrical components:

Lifetime (in hours)	0-20	20-40	40-60	60-80	80-100	100-120
Frequency	10	35	52	61	38	29

Determine the modal lifetimes of the components.

Answer :

Class Interval	Frequencies
0-20	10 f0
20-40	35 f1
40-60	52 f2
60-80 (l)	61
80-100	38
100-120	29

Since the maximum frequency 61 is in the class 60-80, this is the required modal class.

Modal class frequency, f₁ = 61.

Frequency of the class preceding the modal class f₂ = 52

Frequency of the class succeeding the modal class f₂ = 38

Lower boundary of the modal class, l = 60

Height of the class, h = 20