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of tests, and analyses of the test results. Tests are devised to verify all safety requirements where testing has
been selected as appropriate verification method. This is not considered here as analysis. Analysis before
the fact should, as a minimum, consider test coverage for safety critical Must-Work-Functions.
J.7.1 Test Coverage
For small pieces of code it is sometimes possible to achieve 100% test coverage (i.e., to exercise every
possible state and path of the code). However, it is often not possible to achieve 100 % test coverage due to
the enormous number of permutations of states in a computer program execution, versus the time it would
take to exercise all those possible states. Also there is often a large indeterminate number of environmental
variables, too many to completely simulate.
Some analysis is advisable to assess the optimum test coverage as part of the test planning process. There is
a body of theory that attempts to calculate the probability that a system with a certain failure probability will
pass a given number of tests.
Techniques known as “white box” testing can be performed, usually at the modular level.
Statistical methods such as Monte Carlo simulations can be useful in planning "worst case" credible
scenarios to be tested.
J.7.2 Test Results Analysis
Test results are analyzed to verify that all safety requirements have been satisfied. The analysis also verifies
that all identified hazards have been eliminated or controlled to an acceptable level of risk. The results of
the test safety analysis are provided to the ongoing system safety analysis activity. All test discrepancies of
safety critical software should be evaluated and corrected in an appropriate manner.
J.7.3 Independent Verification and Validation
For high value systems with high-risk software, an IV&V organization is usually involved to oversee the
software development. The IV&V organization should fully participate in the validation of test analysis.
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System Safety Handbook系统安全手册下(142)
