Rule-Based Test Techniques

Decision table testing with minimization, review criteria and the checksum procedure, and metamorphic testing.

5 minutes 5 Questions

In the ISTQB Certified Tester Advanced Level Test Analyst syllabus (v4.0), rule-based test techniques are black-box techniques that derive test cases from the business rules, logical conditions, or relationships that govern a test object's behavior. They sit alongside data-based techniques (such as…

Concepts covered
The Checksum Procedure for Decision TablesMetamorphic Testing for Non-Functional Testing and AI-Based SystemsDecision Table Coverage and Implementing Test Cases from RulesMetamorphic RelationsSource Test Cases and Follow-Up Test CasesMetamorphic Testing and the Test Oracle ProblemReviewing Decision Tables: Consistency, Completeness and Overlapping RulesMinimizing Decision Tables with Don't-Care ValuesCombining Metamorphic Testing with Random TestingFull Decision Tables and Infeasible Rules
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CTAL-TA - Rule-Based Test Techniques Example Questions

Test your knowledge of Rule-Based Test Techniques

Question 1

A test analyst checks a draft minimized decision table for an order handling service. The conditions are customer type (retail, business, government), payment (card, invoice) and express (Y, N). Government customers can never pay by card, so those combinations are infeasible. The draft rules are: R1: type=retail, payment=card, express=- -> standard; R2: type=business, payment=-, express=Y -> priority; R3: type=government, payment=invoice, express=- -> manual approval; R4: type=retail, payment=invoice, express=N -> credit check. The analyst plans one test case for each concrete combination that the draft rules match, and will measure decision table coverage against the feasible columns of the full table. Which result is correct for the draft checksum, the checksum target and the coverage these test cases reach?

Correct Answer: Checksum of 7 against a target of 10, giving 70% decision table coverage

The full table has 3 x 2 x 2 = 12 columns. Removing the two infeasible government card columns leaves 10. Each don't-care entry doubles the columns a rule matches, so the draft rules cover 2 + 2 + 2 + 1 = 7. Seven tests against 10 feasible columns give 70% coverage.
A checksum of 10 ignores the gaps: business with express N and retail invoice with express Y are not covered. A target of 12 wrongly counts the infeasible columns. A checksum of 4 counts rules instead of expanding their don't-care entries.
Key concept: decision table checksum validation and coverage measured against feasible columns.

Question 2

A test analyst derives tests from a minimized decision table for a car rental pricing function. The conditions are C1 driver under 25 (Y/N), C2 premium car requested (Y/N) and C3 young-driver waiver held (Y/N). A business constraint states that only drivers under 25 can hold the waiver, so C1=N with C3=Y is infeasible. The rules are: R1: C1=N, C2=-, C3=- -> standard rate; R2: C1=Y, C2=Y, C3=- -> rental refused; R3: C1=Y, C2=N, C3=Y -> standard rate; R4: C1=Y, C2=N, C3=N -> young-driver surcharge. Each test case below is written as C1/C2/C3 followed by its expected result. Which set of test cases achieves 100% decision table coverage of the minimized table, uses only feasible inputs and states correct expected results?

Correct Answer: N/N/N standard; Y/Y/Y refused; Y/N/Y standard; Y/N/N surcharge

The correct set exercises each of the four rules exactly once with feasible inputs: N/N/N for rule 1 (C3=N respects the waiver constraint), Y/Y/Y for refusal, Y/N/Y for the waiver standard rate and Y/N/N for the surcharge, all with correct expected results.
The set starting with N/Y/Y uses an infeasible combination, since a driver aged 25 or over cannot hold the waiver. The set expecting Y/Y/Y to give a standard rate states the wrong result, because a premium car for a young driver is refused regardless of the waiver. The set with two refusal cases never covers the rule for a young driver with a waiver and no premium car.
Key concept: decision table coverage of minimized rules while respecting infeasible condition combinations.

Question 3

A test analyst checks a draft minimized decision table for an order fee service. The conditions are customer type (regular, premium, staff), payment (card, invoice) and express (Y, N). All 12 combinations are feasible. The requirement says staff pay no fee and premium customers pay 5. Regular customers pay 10 by card and 12 by invoice, except that regular express invoice orders cost 15. The draft rules are: R1: type=staff, payment=-, express=- -> fee 0; R2: type=premium, payment=-, express=- -> fee 5; R3: type=regular, payment=card, express=- -> fee 10; R4: type=regular, payment=invoice, express=- -> fee 12; R5: type=-, payment=invoice, express=Y -> fee 15. Which revision makes the table match the full table's checksum and keeps every rule consistent with the requirement?

Correct Answer: Restrict R5 to type=regular and change R4 to express=N

Limiting the 15 fee rule to regular customers and narrowing the 12 fee rule to non-express gives column counts of 4+4+2+1+1 = 12, so every combination is covered exactly once with the required fee.
Keeping the 12 fee rule unrestricted gives a checksum of 13, and the regular express invoice order matches two rules with conflicting fees. Deleting the 15 fee rule does give a checksum of 12, but the express invoice surcharge for regular customers is lost. Keeping the 15 fee rule unrestricted charges staff and premium express invoice orders 15, and it leaves their express card orders uncovered.
Key concept: decision table minimization and checksum verification.

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