Question

To check the quality of two brands of light bulbs, their life in burning hours was estimated as under for 100 bulbs of each brand.

Life		No. of bulbs
(in hrs)		Brand A	Brand B
0 – 50		15	2
50 – 100		20	8
100 – 150		18	60
150 – 200		25	25
200 – 250		22	5
		100	100

(i) Which brand gives higher life?

(ii) Which brand is more dependable?

Answer 1

For Brand A
Life (in hours)	No. of bulbs	M	A = 125		d'2	fd'	fd'2
	f	x	X – A = d
0 – 50	15	25	– 100	– 2	4	– 30	60
50 – 100	20	75	– 50	– 1	1	– 20	20
100 – 150	18	125	0	0	0	0	0
150 – 200	25	175	50	1	1	25	25
200 – 250	22	225	100	2	4	44	88

For Brand B
Life (in hrs)	No. of bulbs (f)	Mid value m	A = 125		d'2	fd'	fd'2
0 – 50	2	25	– 100	– 2	4	– 4	8
50 – 100	8	75	– 50	– 1	1	– 8	8
100 – 150	60	125	0	0	0	0	0
150 – 200	25	175	50	1	1	25	25
200 – 250	5	225	100	2	4	10	20

(i) The average life of bulb of Brand B has comparatively higher life than the bulb of Brand A.

(ii) The bulbs of Brand B is more dependable as CV of Brand B is lesser than CV of Brand A.