Requirements traceability matrix and bidirectional tracing
Bidirectional traceability is the ability to trace any requirement up to its parent requirement or expectation and down to the child requirements allocated from it. NASA records this in a requirements matrix or modeling tool as each requirement is written, and uses it to catch gold plating and to show every requirement has been verified.
What gets traced
NASA defines traceability as a discernible association between two or more items such as requirements, system elements, verifications or tasks. Requirements management keeps bidirectional traceability between stakeholder expectations, customer requirements, technical requirements, component requirements, design documents and test plans.
The trace starts early. Well-written needs, goals and objectives trace from needs to goals to objectives. A goal that supports no need, or an objective that supports no goal, should not be in the set.
Recording the trace
As each requirement is documented, record its bidirectional traceability. Trace each requirement to a parent requirement or expectation in a baselined document, or mark it as self-derived and seek concurrence from the next higher level. Self-derived requirements include locally adopted good practices and results of design decisions.
Check the trace, independently if possible, to make sure it is correct and fully covers its parent. Make sure every top-level requirement has been allocated to lower levels. Traceability is usually recorded in a requirements matrix or a requirements modeling application.
Orphans, gold plating and verification
If a requirement has no parent and is not an acceptable self-derived requirement, either the traceability process is flawed and should be redone, or the requirement is gold plating and should be eliminated.
Later, product verification and validation results are mapped into the requirements database with the goal of showing every requirement verified and validated. All requirement changes go through review and approval to keep the trace intact.
Where the exam catches people
Believing traceability runs only one way, from requirement to test: NASA requires bidirectional tracing, up to parents and down to children.
Believing a requirement with no parent should simply be given one: if it is not acceptably self-derived, the trace process is redone or the requirement is removed as gold plating.
Believing tracing is done once at the end: the trace is recorded as each requirement is documented.
Believing an objective can stay in the set without a parent goal: NASA says it should not be part of the integrated set.