The four types of verification can be used at any of the levels, although some work better than others for a given level of verification. As an example, the most effective way to find anomalies at the component level is inspection. On the other hand, inspection is not applicable at the system level (you don't look at the details of code when performing system level testing). A logical approach to testing is to utilize techniques and methods that are most effective at a given level.
Component level verification can easily get very expensive. Companies need to avoid making statements like "all paths and branches will be executed during component testing." These statements make for a very expensive test program, as all code developed is required to have one of the most labor-intensive type of testing performed on it. To minimize the costs of component verification, the V&V group develops rules for determining the type of verification method(s) needed for each of the software functions. As an example, very low complexity software function, which is not on the safety critical list, may only need informal inspection (walkthrough) performed. Other complicated functions typically require white box testing since the functions become difficult to determine how they work. We recommend performing inspections before doing the white box testing for a given module as it is less expensive to find the errors earlier in the development.
The FDA is embracing V&V as the primary way of proving your system does what you intended and also meets the needs of the people using it. The resulting V&V effort has become a significant part of the software development effort for a medical device. One of the key pieces to demonstrate that the system is implemented completely is a Requirements Traceability Matrix (RTM), which documents each of the requirements traced to design items, code, unit, integration, and software system test. The RTM is an easy and effective way for documenting your implementation in a manner to which the FDA expects the medical device manufacturer to be able to answer:
|• What are the requirements?
||• Where are they implemented?
||• How have you tested them?