The data presented in the table for Problem 3.30 are known as grouped data, of the type that are developed in order to plot a frequency histogram. The mean of such data can be determined as a weighted average of the class marks, as follows:

Where and are the frequency and class mark, respectively, for k class intervals.
(a) Demonstrate that the mean of the interevent times of Problem 3.30 is as stated.
(b) How many interevent time values were used in the analysis?
In Problem 3.30
Interevent Time
(Days)Frequency
0-1 ........................... 35
1-2 ........................... 12
2-3 ............................ 6
3-4 ............................ 6
4-5 ............................ 7
5-6 ............................ 3
6-7 ............................ 1
7-8 ............................ 1
8-9 ............................ 3
9-10 ........................... 2
10-11 ......................... 1
11-12 ......................... 0
12-13 ......................... 0
13-14 ........................ 10

i=- i=l =1 II