Reduce test cases by first deciding what behavior and risk the suite must still cover, then choose the right method: minimize redundant tests in the suite, select tests relevant to a code change, or prioritize tests so the most useful feedback arrives first. A lower test count alone does not show that a suite is better or still adequate.
Start by defining what the suite must protect
Before removing or rearranging tests, state the requirements, user-visible behaviors, and coverage obligations the suite serves. Where possible, map each test to the requirement or behavior it checks. Keep that traceability when cases are removed, so the remaining suite still has an explicit purpose.
Do not treat line coverage as the sole measure of a test’s value. Two tests with similar code coverage can exercise different boundaries, states, or interactions. Review the consequences of a missed fault as well as the tests’ overlap.
NIST IR 8397 recommends a varied verification approach, including automated, black-box, structural, historical, and fuzz testing. It is minimum, broadly applicable guidance, not an exhaustive verification standard. NIST describes its scope this way: “The document does not address the totality of software verification, but instead recommends techniques that are broadly applicable and form the minimum standards.” NIST IR 8397 was published October 6, 2021, by Paul E. Black, Vadim Okun, and Barbara Guttman.
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Choose the right kind of test-suite reduction
“Making the suite smaller” can mean three different things. Minimization changes the retained suite; selection changes which tests run for a particular modification; prioritization changes the run order. Yoo and Harman’s survey treats these as distinct ways to address the cost of regression suites as software evolves. NASA SWE-191
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Remove redundant cases without losing their purpose
- Write down the criterion. For example, decide whether you are preserving requirement coverage, structural coverage, or a particular set of behavior and interaction checks. “Keep the important tests” is not precise enough to evaluate removals.
- Map cases to what they exercise. Link tests to requirements, behaviors, relevant code, input boundaries, state transitions, and important configurations where that information is available.
- Identify overlap, then inspect the differences. Cases that appear to cover the same behavior may use different starting states, boundary values, error conditions, or combinations of settings. Confirm those distinctions before deleting anything.
- Remove or consolidate only against the stated criterion. Preserve at least one test for every obligation you intend to retain. Keep the mapping current so future changes do not silently erase coverage.
- Recheck the resulting suite. Verify that the retained tests still address the declared obligations and that the change has not removed a distinct failure mode. Revisit the choice when requirements, implementation, or risk changes.
These checks apply whether you are deleting duplicates, consolidating cases, or trying to make a suite easier to maintain. Similarity is a reason to investigate, not by itself evidence that a test has no value.
Select regression tests for a code change carefully
Selection can reduce the tests run for one change without permanently shrinking the full suite. Its usefulness depends on the evidence available to relate the change to tests—for example, traceability between code and tests—and on the consequences of missing a fault.
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- Identify the changed code and the behaviors or requirements it can affect.
- Use the available change-to-test relationships to select a subset, while accounting for dependencies and indirect effects.
- Keep the full regression suite available for situations where the selection conditions are not established or the impact of a missed fault is unacceptable.
- Do not label a subset “safe” merely because it is smaller or ran successfully. Safety depends on the assumptions and conditions behind the selection.
NASA’s handbook frames safe regression selection as choosing a subset that, under defined conditions, excludes no test that would reveal a fault in modified software. That is a condition to establish, not a guarantee provided by selection in general. NASA SWE-191
Use prioritization when the problem is feedback time
If the full suite still needs to run but results arrive too slowly, prioritization changes the order rather than removing cases. Put tests earlier when their results are especially useful to the team’s decision-making, based on the coverage and risk objectives you have defined. The remaining tests still matter even if they run later.
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This distinction prevents a common mistake: treating “we want faster results” as a reason to delete tests when changing the run order could address the immediate need without permanently reducing coverage.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Reduce configuration combinations with interaction testing
When a product has many settings or configurations, exhaustively testing every combination can create a large suite. Combinatorial testing chooses cases to cover interactions among parameter values rather than enumerating the full Cartesian product. The interaction strength should reflect the project’s risks and constraints; results reported for this method are evidence that it can help, not a promise of the same reduction for every system.
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NIST presents combination coverage as a supplement to structural coverage. Its project page reports multiple studies with suite-size reductions of 20X to 700X while detecting faults at a level equal to exhaustive testing. These are results reported across those studies, not a universal benchmark or expected outcome for a particular project. NIST: Combinatorial Methods for Trust and Assurance
A 2024 paper by M. S. Raunak, Richard Kuhn, Raghu Kacker, and Yu Lei describes parameter-interaction coverage and reports reductions of 20x–700x while approaching exhaustive fault detection. The paper appeared in IEEE Reliability Magazine in March 2024; its NIST-hosted record was published February 5, 2024. The wording and scope of reported results differ, so neither figure should be read as a guarantee for an individual test suite. NIST-hosted record: Combinatorial Testing for Building Reliable Systems
Compare approaches against risk and cost
Choose a method by asking what you are trying to reduce and what evidence supports the change.
- Goal: Do you need fewer tests retained, fewer tests for a specific change, or faster feedback through earlier results?
- Coverage target: Are you protecting requirements, code structure, or interactions among parameter values and configurations?
- Change evidence: Can you reliably relate a code change to the tests that exercise affected behavior?
- Missed-fault impact: How likely and costly would an undetected regression be?
- Total cost: Will the reduction save execution and maintenance time without making traceability or diagnosis harder?
These questions synthesize the distinctions in the regression-testing survey, NIST’s combinatorial-testing guidance, and NASA’s regression-testing handbook. They do not replace project-specific risk assessment.
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Troubleshoot a reduction that causes trouble
- A regression appears in a case you removed: Revisit the criterion and test-to-behavior mapping. Restore coverage for the distinct boundary, state, or interaction that the removed test exercised.
- A selected subset misses relevant tests: The change-to-test evidence or selection assumptions were incomplete. Expand the subset or run the full suite when safe-selection conditions cannot be established.
- The suite is smaller but still slow: The reduction may not target the main cost. Determine whether execution time, test setup, maintenance, or slow feedback is the actual problem; prioritization addresses ordering, while selection addresses per-change scope.
- Configuration coverage remains unwieldy: Identify which parameter interactions matter to risk and evaluate combinatorial coverage as a supplement to structural tests, rather than assuming exhaustive combinations are the only option.
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