The "Defect Age" metric is a measure of the time between when a defect is introduced into the system and when it's discovered and resolved. It provides insight into how quickly a team identifies and addresses defects. MTBF (Mean Time Between Failures) measures the expected time between two successive failures of a system.

Usability testing is geared towards understanding how end-users interact with the software and ensuring a positive user experience. The primary considerations involve the system's ease of use, its consistency in design and functionality, and adherence to user interface design guidelines. UAT, on the other hand, determines if the system meets user needs, while load and compatibility have different focuses.

Security testing primarily aims to identify vulnerabilities, weaknesses, and potential threats in a software application. It is crucial for protecting data from unauthorized access and breaches, ensuring the confidentiality, integrity, and availability of the system.

Automated testing involves the initial cost of writing and maintaining scripts, making it not cost-effective for one-time testing scenarios. While automation can handle large-scale projects and performance testing, it's not recommended when the requirements change often.

Test levels, which include stages like unit, integration, system, and acceptance testing, play a critical role in the Test Planning phase. They help in organizing the sequence and scope of testing activities, ensuring that all components of the software are thoroughly evaluated at the appropriate stages.

Exploratory test cases are based on real-time learning and testers' expertise. Automation is not suitable because these tests are often non-repetitive, less predictable, and leverage the human ability to spot unexpected issues in an ad-hoc manner.

Stubs are used in incremental integration testing as placeholders for modules that have not yet been developed. They simulate the behavior of these missing modules and allow testing to continue in the presence of incomplete components.

Regularly updating and maintaining an automated test suite is crucial to ensure that the suite is always in sync with the current version of the application. As software evolves, test cases that were once relevant might become obsolete, and new test scenarios may arise. Maintenance ensures the suite remains effective and reflective of current needs.

Cyclomatic Complexity is a white-box testing metric used to determine the complexity of a program. It calculates the number of linearly independent paths through a program's source code. A higher complexity score can indicate areas of the code that might be riskier or overlooked during testing, thus requiring more thorough testing.

Residual risk is the risk that remains after all risk response, mitigation, or prevention activities have been implemented. It represents the remaining threat even after you've taken measures to reduce the severity or likelihood of the adverse event.