Since problems can wreak havoc with a supply chain and, eventually, to the bottom line, a well-executed Testing Phase can make a big difference to a project. A corollary to Murphy's Law could be if a design element hasn't been tested, then it probably doesn't work. Actually, failure can be far more subtle: a component appears to function fine in the lab, but later malfunctions in the field. The worst problems are the ones that only occur rarely and with no apparent cause.

Often, a Testing Phase is used to discover problems and preempt malfunctions by the time a design is complete. By developing and running suitable tests for the different aspects of a design, a high degree of functionality can be verified. For hardware, longevity tests (Mean Time Between Failures, or MTBF) can provide an estimate of the life of subassemblies and replaceable components. For replaceable components, appropriate maintenance procedures can be developed. For operating issues that arise or are considered strong possibilities, troubleshooting guidelines can be developed.

Software and firmware can be especially difficult to test completely because of the vast combinations of parameters, variables and operating modes. DataPlex uses effective ways of testing, such as employing Monte Carlo simulation, information theory, and quality control methods.

The Testing Phase is not generally considered an independent phase, although it can be in some cases (e.g. post-design failure mode analysis), but one that runs parallel to a Design Phase and concurrent with a Patents Phase for when intellectual property development is underway.

Any serious problems or limitations discovered during the Testing Phase are reviewed to everyone's satisfaction, which may sometimes necessitate another Design Phase iteration. After a successful Testing Phase comes the Production Phase.