Question

The number of telephone calls received at an exchange per interval for 250 successive one-minute intervals are given in the following frequency table.

No. of calls (x):	0	1	2	3	4	5	6
No. of intervals (f):	15	24	29	46	54	43	39

Compute the mean number of calls per interval.

Answer 1

Let the assumed mean (A) = 3

Number of calls (xi)	Number of intervals (fi \(\phi\))	ui = xi - A = xi - 3	fiui
0	15	-3	-45
1	24	-2	-48
2	29	-2	-48
3	46	0	0
4	54	1	54
5	43	2	86
6	39	3	117
	N = 250		\(\sum\)fiui = 135

Mean number of calls = A + \(\frac{\sum f_iu_i}{N}\)

= 3 + \(\frac{135}{250}=\frac{885}{250}\)

= 3.54