Phishing attacks to e-mail accounts. Refer to the Chance

(Summer 2007) article on phishing attacks at a company, Exercise 2.47

(p. 81). Recall that phishing describes an attempt to extract personal/financial information through fraudulent e-mail. The company set up a publicized e-mail account—called a “fraud box”—that enabled employees to notify it if they suspected an e-mail phishing attack. If there is minimal or no collaboration or collusion from within the company, the interarrival times (i.e., the time between successive e-mail notifications, in seconds) have an approximate exponential distribution with a mean of 95 seconds.

a. What is the probability of observing an interarrival time of at least 2 minutes?

b. Data for a sample of 267 interarrival times are saved in the PHISH file. Do the data appear to follow an exponential distribution?