Page 149 - Safety Risk Management for Medical Devices

128 Safety Risk Management for Medical Devices

Figure 15.1 Contribution of software to Hazards.

Of the above types of software only 3.b is not safety related, and the rest should be
included in the software risk management process.
As depicted in Fig. 15.1, Hazards are the consequence of a chain of events.
Exposure to Hazards can cause Harm. Since exposure to software cannot cause Harm,
software itself is not a Hazard. But software failures can cause Hazards in the System
context. To determine the risks due to software, we’ll need to identify the Harms that
could result from software failures.
Understanding the contribution of software to Hazards and Harms can be
achieved via top-down System analyses, e.g., Fault Tree Analysis. This requires
knowledge of the System architecture, indication for use, and the context within
which the System operates. Starting with potential Harms of the System deductively
analyze the pathways which could lead to those Harms. If no pathway contains soft-
ware, you can conclude that software in that System is not a contributor to Hazards.
If it is established that software is a contributor to Hazards and device risks, it is ben-
eficial to analyze the software architecture to determine the roles and contributions of
the various software items to risk. For simple devices, the manufacturer may choose to
treat the software as a black box and bypass the software architectural analysis.
Before we proceed, it’s important to understand some relevant vocabulary. Three
important terms are:
Software System An integrated collection of Software Items organized to accomplish a
specific function or set of functions [9]
Software Unit Software Item that is not subdivided into other items [9]
Software Item Any identifiable part of a computer program, i.e., source code, object
code, control code, control data, or a collection of these items [9]

All levels of software composition can be called software Item,
including the top-level: Software System, and the bottom-level:
Software Unit