Quality in Design - Medical Device Design: Innovation from Concept to Market - page 143

CASE STUDY 7.4
Let us consider a simple computer program which is used to support the use of a device. It is
intended, due to its complexity and size, to run the software on a web-based server and the
end-user has access to it over the Internet. Without this software the device cannot be used.
One obvious failure mode is that the program stops working. Let us look at the first level of
events that may have caused this. The first is that there is an error in the program that causes
it to “lock”; the second is that the host server (the Internet provider) fails; the third is that your
server fails; and the fourth is that your computer fails ( Figure 7.11 ).
Whatever the failure mode, the overall outcome is that the user will see it as your fault . All
potential issues are important. Can you see that our piece of software is reliant on other
people's equipment - hence if the host computer's wireless connection is faulty do you think
the surgeon or clinician is going to hold your software blameless just because it was a hardware
problem? No, the procedure is still cancelled - you will still be in the wrong. Hence we use fault
tree analysis to imagine what can happen in the overall system.

Program
stops
working

Host
server
failure

Local
computer
fails

Local
Internet
interuption

Program
failure

Local
Int ernet
connection
failu re

Host
computer
Internet
failure

Host
computer
Internet
component
fault

Wireless
connection
lost

Internet cable
disconnected

Weak
signal

Host computer
wireless
interface
fault

Figure 7.11
Example fault tree for an Internet-based program failure.