Usability testing or evaluation provides a trusty evaluation, because it assesses usability through samples of real users. However, it has a number of drawbacks, such as the difficulty to properly select a correct sample of the user community, and to train it to manage not only the main application features but also the most sophisticated and advanced facilities of an interactive system.
With respect to Usability testing or evaluation, usability inspection methods are more subjective, having heavy dependence upon the inspector skills.
The main advantage of inspection methods is however the cost saving: they do not involve users nor require any special equipment or lab facilities. In addition, experts can detect a wide range of problems and possible faults of a complex system in a limited amount of time. For these reasons, inspection methods have achieved widespread use in the last years, especially in the industrial environments, since industry is very much interested in effective methods that can provide good results being still cost-effective.
Inspection methods aim at finding usability problems in an existing user interface, and make recommendations for fixing these problems. Hence, they can be applied at various steps of the software development, and are certainly used for evaluating the design of the system in a prototype form, even a paper prototype, so that possible defects can be fixed as soon as possible. When a system implementation is available, user-based evaluation is often recommended.