Quality in Design - Medical Device Design: Innovation from Concept to Market - page 145

Program
stops
working

Host
server
failure

Local
Internet
interuption

Host
computer
fails

Program
failure

1/312.5

Local
Internet connection
failure

Host computer
Internet failure

Wireless
connection
lost

Host computer
Internet
component fault

Internet cable
disconnected

1/1000 1/500 1/5000

Weak signal

Host computer
wireless
interface
fault

Figure 7.12
Fault tree using standard symbols.

OR logic, in other words any one of these will result in a loss of local Internet connection. If we
assume the probability of losing the wireless connection is 1/500, and the probability of the host
computer's hardware being faulty is 1/5000, then we can determine the overall probability of the
program stopping because of a loss of local Internet connection. If we assume that it is unlikely
that all three occur simultaneously, then the probability of one of them happening is given by

Probability of either A or B occurring:

(7.6)

PA BPAPB ( ) () ()

If they can happen at the same time then Equation (7.6) becomes

(7.7)

PA BPAPBPAPB ( ) () () ()()