Fault Removal

Fault Removal

The old saying "an ounce of prevention is worth a pound of cure" is nowhere more true than in fault removal. Isolating a fault once it is incorporated into a delivered system is from 10 to 100 times more costly than preventing it in the first place. It has been the authors experience that on a commercial, multimedia software product included as part of Microsoft Windows, the cost to isolate and remove a fault consisting of three lines of assemble code is $60,000. The cost to have prevented this fault was one code inspection lasting two hours with four participants. Eight person-hours would have cost less than $1,000. The cost to prevent was determined from the time it took to isolate the fault once discovered in the released operating system and analyzing the development process of the driver code at fault. The final code level inspection was skipped because of time pressures for this driver release to Microsoft.

Fault removal begins at the first opportunity that faults injected into the product are discovered. At the design phase, requirements phase products are passed to the design team. This is the first opportunity to discover faults in the requirements models and specifications. Fault removal extends into implementation and through installation. For a product being developed for use by an internal client, such as a new human resources system or an electronic commerce Web page, the installation may be just one point in time. For commercial products, the installation occurs every time a new purchaser breaks the shrink-wrap and begins the installation process on their personal computer or company server. The project and product manager must be aware of the temporal nature of the installation phase and gauge their reliability efforts accordingly.

The first portion of fault removal is design and implementation, as described previously in the section on fault prevention. The second portion, installation, begins with work that should have been done during the requirements phase. Realistically, project managers do not need an operational profile defined until there is a partially functioning system to operate. This can be done earlier in the product development life cycle through the use of prototypes.

Building on work done in functional profiling, the next step in the installation portion is to conduct reliability growth testing. This is also referred to as load testing. Based on the functions to be performed, functional profile, the way those functions are executed, and operational profile, a set of testing scenarios are defined and executed that progressively push the system past its defined operational regime. The goal of reliability growth testing is to determine the mix of loads at which the system will break. This is a formal process where tracking of testing progress is critical. The results of the tests are analyzed and used to recalibrate the models used in reliability forecasting early in the project. One of the duties of a project manager is to support continuous process improvement. Taking data from one project and using it to refine the tools and techniques for future projects supports the organization's ability to learn.

During the middle phases of the software development life cycle, reliability efforts focus on defect removal. Table 1 shows the subprocesses of the major design, implementation, and installation life cycle phases that support defect removal.

Fault Removal Life Cycle Activities

The projection of additional testing required is a result of the analysis of the testing data. Load or stress testing may not be adequate to certify that all reliability objectives are met. Some modules may have to go through white- or black-box testing again. Regression testing may have to be increased and expanded. At the end of the fault removal, the project manager must be satisfied with the results and certify that reliability objectives have been met.


operating system, fault removal, life cycle
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