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Black-Box Test Techniques Interview Questions for ISTQB

25 Black-Box Test Techniques questions with answers for istqb roles — 7 easy, 11 medium, 7 hard.

About Black-Box Test Techniques

Practise the specification-based techniques that carry the most K3 exam marks: equivalence partitioning with Each Choice coverage, 2-value and 3-value boundary value analysis, decision table testing with minimisation, and state transition testing with all states, valid transitions and all transitions coverage. Solve exam-style counting and coverage problems step by step.

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01

Which black-box test techniques are covered in CTFL v4.0?

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CTFL v4.0 covers four black-box techniques: equivalence partitioning, boundary value analysis (2-value and 3-value), decision table testing and state transition testing. They derive tests from the specification without looking at the internal structure, so tests stay valid even when the implementation changes. Use case testing, which was in older syllabi, is no longer part of the v4.0 Foundation syllabus.
02

Explain equivalence partitioning and its coverage.

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Equivalence partitioning divides data into partitions whose values are expected to be processed the same way, so one value per partition is enough. Partitions can cover inputs, outputs, configuration items or time, and include valid and invalid partitions. They must be non-empty and must not overlap. Coverage is partitions exercised divided by partitions identified. With several inputs, Each Choice coverage requires every partition of every input to be used at least once.
03

An age field accepts 18 to 60. Apply equivalence partitioning. How many tests give 100 percent EP coverage?

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There are three partitions: below 18 (invalid), 18 to 60 (valid) and above 60 (invalid). Pick one value from each, for example 10, 35 and 70. So three test cases give 100 percent EP coverage. Test invalid partitions one at a time so that one invalid value does not hide another, and confirm with the specification whether non-numeric input forms another invalid partition.
04

A quantity field accepts 1 to 100. List the 2-value BVA test values.

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The valid partition is 1 to 100, with invalid partitions below and above it. For each boundary, take the boundary and its closest neighbour in the adjacent partition. Lower boundary: 0 and 1. Upper boundary: 100 and 101. So 2-value BVA needs four values: 0, 1, 100 and 101, and four test cases give 100 percent 2-value boundary coverage.
05

A password must be 8 to 16 characters long. What are the 2-value BVA tests?

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The partitions by length are 0 to 7 (invalid), 8 to 16 (valid) and 17 or more (invalid). 2-value BVA takes each boundary and its closest neighbour: 7 and 8 for the lower boundary, 16 and 17 for the upper boundary. So test passwords of length 7, 8, 16 and 17 characters, keeping other rules such as character types valid so only the length varies.
06

Discounts depend on several combined business rules such as membership, coupon and festival offers. Which technique do you choose?

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Decision table testing fits best, because the outcome depends on combinations of conditions. I would list the conditions and actions, build the full table, remove infeasible combinations, minimise columns where a condition does not matter, and turn each column into a test case. The table also exposes gaps or contradictions in the rules, which I would raise with the business analyst before testing.
07

A field accepts integers from 1 to 99. Which set of values achieves 100 percent 2-value BVA coverage: {1, 99}, {0, 1, 99, 100}, {0, 50, 100} or {1, 2, 98, 99}?

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The answer is {0, 1, 99, 100}. Two-value BVA needs each boundary value and its closest neighbour in the adjacent partition: 0 and 1 at the lower edge, 99 and 100 at the upper edge. {1, 99} misses the invalid neighbours, {0, 50, 100} misses 1 and 99, and {1, 2, 98, 99} stays inside the valid partition, which is closer to 3-value thinking.
08

What is the difference between 2-value and 3-value boundary value analysis?

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BVA applies to ordered partitions. In 2-value BVA each boundary has two coverage items: the boundary value and its closest neighbour in the adjacent partition. In 3-value BVA there are three: the boundary and both neighbours. For a valid range 18 to 60, 2-value gives 17, 18, 60, 61, and 3-value gives 17, 18, 19, 59, 60, 61. 3-value is stricter and can catch defects like 'x == 18' written instead of 'x >= 18'.
09

How does decision table testing work, and how is coverage measured?

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Decision tables model combinations of conditions and the resulting actions, which suits business rules. Each column is a rule: one combination of conditions with its actions. With n Boolean conditions, a full table has 2 to the power n columns. Coverage items are the feasible columns, so coverage is exercised columns divided by feasible columns. Tables can be minimised by merging columns where a condition does not affect the outcome, shown with a dash.
10

A field accepts integers from 5 to 15. List the 3-value BVA test values.

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In 3-value BVA, each boundary gives the boundary value plus both neighbours. Lower boundary 5 gives 4, 5 and 6. Upper boundary 15 gives 14, 15 and 16. So the six values are 4, 5, 6, 14, 15 and 16. Compared with 2-value BVA (4, 5, 15, 16), the extra values 6 and 14 help detect mistakes in comparison operators inside the range.
11

Payment type has 3 partitions (card, UPI, wallet) and customer type has 2 (new, existing). What is the minimum number of tests for Each Choice coverage?

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Each Choice coverage needs every partition from each input used at least once, and one test can cover one partition of each input at the same time. So the minimum equals the largest number of partitions: 3. Example tests: card with new, UPI with existing, wallet with new. Covering all combinations would need 3 x 2 = 6 tests, which is not required for Each Choice.
12

Build a decision table for a loan rule: approve if salary is at least 30,000 and credit score is at least 700; refer if only one is met; otherwise reject.

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Conditions R1 R2 R3 R4 Salary >= 30000 T T F F Score >= 700 T F T F Actions Approve X - - - Refer to officer - X X - Reject - - - X Two Boolean conditions give 2 x 2 = 4 rules, so 4 test cases give 100 percent decision table coverage.
13

Marks from 0 to 100 give: 0-39 fail, 40-59 pass, 60-100 distinction. How many partitions and 2-value boundary values are there?

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Valid partitions: 0-39, 40-59 and 60-100, plus two invalid partitions: below 0 and above 100, so 5 partitions. Boundaries in 2-value BVA: -1, 0, 39, 40, 59, 60, 100 and 101, which is 8 values. Five tests give 100 percent EP coverage, and eight tests give 100 percent 2-value boundary coverage. Note that output partitions like fail, pass and distinction also guide the partitioning.
14

Free shipping applies for orders of 499 rupees or more, otherwise a fee of 40 applies. Design tests using EP and BVA.

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Partitions: 0 or less is invalid, 1 to 498 pays 40, and 499 or more ships free. EP tests: 0 rejected, 250 pays 40, 800 free. 2-value BVA adds 0 and 1 at the lower edge and 498 and 499 at the free shipping boundary. Combined minimal set: 0, 1, 250, 498, 499 and 800, each with the expected fee clearly stated.
15

Discount depends on customer type (regular, gold, platinum) and whether the order exceeds 1,000 rupees. How many rules does the full decision table have?

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When conditions are not Boolean, multiply the number of values of each condition. Customer type has 3 values and the order condition has 2, so the full table has 3 x 2 = 6 rules. Each rule becomes one test case for 100 percent decision table coverage. If, for example, platinum always gets the same discount regardless of amount, those two columns can be merged, giving 5.
16

An order can be placed, shipped, delivered, cancelled or returned. How would you test this workflow?

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State transition testing fits because behaviour depends on the current state and events. I would draw the diagram, for example placed to shipped, shipped to delivered, placed to cancelled, delivered to returned, and build a state table. Tests cover every valid transition and attempt invalid ones, such as cancelling a delivered order or returning an undelivered one, checking the system rejects them correctly.
17

A developer says one value per partition is enough. Why might you still add boundary tests?

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Equivalence partitioning assumes all values in a partition behave the same, but many defects occur at the edges, where developers write '>' instead of '>=' or use the wrong limit. One middle value like 35 in an 18 to 60 range will never reveal these. Boundary value analysis adds tests at 17, 18, 60 and 61, so the two techniques together give good coverage with few tests.
18

Tell me about a time a test design technique helped you find a defect.

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Situation: I was testing a scholarship form where students aged 17 to 25 were eligible. Task: design efficient tests for the age rule. Action: I applied boundary value analysis and tested 16, 17, 25 and 26. Result: 25 was wrongly rejected because the code used 'less than 25'. The developer fixed it, and I shared the BVA checklist with the team so others used it too.
19

What are the coverage criteria for state transition testing?

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All states coverage requires every state to be visited. Valid transitions coverage, also called 0-switch coverage, requires every valid transition to be exercised. All transitions coverage requires every valid transition to be exercised and every invalid transition to be attempted, ideally one invalid transition per test to avoid fault masking. Full valid transitions coverage guarantees full all states coverage, and all transitions coverage is recommended for safety-critical software.
20

A decision table has three Boolean conditions. If condition A is false, the result is always 'reject'. How many columns does the minimised table have?

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A full table has 2 to the power 3 = 8 columns. When A is false, the other two conditions do not matter, so those four columns merge into one column with dashes for B and C. When A is true, the four combinations of B and C remain. The minimised table has 4 + 1 = 5 columns, so 5 tests give full coverage, if all columns are feasible.
21

Model an ATM PIN entry with three attempts as states and transitions. How many tests are needed for valid transitions coverage?

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States: Idle, WaitPIN, Try2, Try3, Access, Locked Idle --insert card--> WaitPIN WaitPIN --correct PIN--> Access WaitPIN --wrong PIN--> Try2 Try2 --correct PIN--> Access Try2 --wrong PIN--> Try3 Try3 --correct PIN--> Access Try3 --wrong PIN--> Locked There are 7 valid transitions. Access and Locked are final states, so each test ends there; covering all 7 needs at least 4 tests.
22

How do you use a state table to find invalid transitions?

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A state table lists states as rows and events as columns, and each cell shows the resulting state or marks the transition as invalid. Example: a document workflow with states Draft, Review and Published, and events submit, approve and reject. Draft with approve is invalid. Valid cells are coverage items for valid transitions coverage; invalid cells are tested for all transitions coverage, one per test case.
23

You identified 6 equivalence partitions and your tests exercise 4 of them. What is the coverage? Does adding a second test in the same partition help?

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EP coverage = exercised partitions / identified partitions x 100 = 4 / 6 x 100 = 66.7 percent. Adding another test in an already covered partition does not increase coverage, because coverage counts partitions, not tests. To reach 100 percent you need tests in the two missing partitions. Exams often include such distractors to see if you count tests instead of coverage items.
24

An amount field accepts 0.01 to 999.99 with two decimal places. What are the 2-value boundary values?

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The closest neighbour depends on the precision, here 0.01. Lower boundary: 0.00 (invalid) and 0.01 (valid). Upper boundary: 999.99 (valid) and 1000.00 (invalid). So the four test values are 0.00, 0.01, 999.99 and 1000.00. Always confirm the precision from the specification, because with whole numbers the neighbour would be 1, and with three decimals it would be 0.001.
25

An insurance premium depends on five Boolean conditions. The full decision table has 32 columns. How do you keep testing manageable?

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First remove infeasible combinations, for example 'under 18' with 'retired'. Then minimise the table by merging columns where a condition has no effect on the action. Next, prioritise the remaining rules by risk and business value, testing high-impact rules first. I would also verify the rules with the business, since a large table often exposes contradictions or missing actions.

Frequently asked questions

How important is Black-Box Test Techniques in ISTQB interviews?

Practise the specification-based techniques that carry the most K3 exam marks: equivalence partitioning with Each Choice coverage, 2-value and 3-value boundary value analysis, decision table testing with minimisation, and state transition testing with all states, valid transitions and all transitions coverage. Solve exam-style counting and coverage problems step by step.

How should I use the Black-Box Test Techniques sheet?

Go top to bottom — easy to hard. Answer before opening each answer, tick what you solved and save misses to Last-Minute Revision.

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