Design test cases by first identifying what shapes the behavior: input classes, ordered limits, combinations of rules, or changes over time. Use equivalence partitioning for classes, boundary value analysis for ordered limits, decision tables for interacting conditions, and state-transition testing for history-dependent behavior. These techniques complement one another; none is a complete test strategy.
How do I design test cases systematically?
- Read the requirement. Identify observable behavior, constraints, business rules, and any meaningful states or events.
- Choose a model. Use partitions for groups expected to behave alike, boundaries for ordered limits, decision tables for combinations of conditions, or state transitions when behavior depends on history.
- Write down the model before choosing test data. List the partitions, boundary values, rule combinations, or transitions you intend to cover.
- Turn model elements into cases. For each case, record its preconditions, input or event, expected result, and the requirement or model element it checks.
- Review gaps. Look for omitted invalid inputs, untested condition combinations, unreachable states, and adjacent boundary values that matter.
- Add other perspectives where needed. Structural and experience-based approaches can reveal risks that specification-derived techniques may miss; select them in light of the system, risk, requirements, standards, and team skill.
A test design technique derives tests from a basis or model. It is not, by itself, a complete testing strategy.
Which test design technique fits the problem?
| Technique | Use it when | What to design tests around | Review question |
|---|---|---|---|
| Equivalence partitioning | Inputs, outputs, internal values, time values, or interface parameters form groups expected to be processed alike. | Representative values from relevant valid and invalid partitions. | Are the classes meaningful, and what assumptions make their members equivalent? |
| Boundary value analysis | Partitions are ordered and behavior may fail at their limits. | Boundary values and adjacent values, using a two-value or three-value variant. | Which edges are included, and does the requirement say the limit is inclusive? |
| Decision table testing | Different combinations of conditions lead to different outcomes. | Condition combinations and the actions or outcomes each rule specifies. | Which combinations are relevant, and can rules be simplified without losing required coverage? |
| State-transition testing | The system has meaningful states and events, and current state affects behavior. | State changes and paths through the model. | What state, transition, or path coverage matters for the risks in scope? |
These are not competing universal methods. Choose based on the test basis, likely defects, expected behavior, and coverage goal. A single feature may need more than one model.
Equivalence partitioning: test representative classes
Equivalence partitioning divides values into groups expected to receive the same treatment. Select representative values from the relevant valid and invalid partitions rather than treating every possible value as a separate design problem. The assumption that members of a partition behave alike is a test-design heuristic, not proof that every value in the group is defect-free.
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Partitions may describe inputs, outputs, internal values, time values, or interface parameters. Make the expected treatment explicit: a partition is useful only when its members are expected to follow the same behavior for the requirement being tested.
Boundary value analysis: test the edges
Boundary value analysis applies to ordered partitions, where adjacent values may be handled differently. The ISTQB Foundation Level material includes two-value and three-value variants. A two-value selection checks a boundary and an adjacent value outside it; a three-value selection checks below, on, and above a boundary. State which variant you are using so the intended coverage is clear.
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Example: age from 18 through 120 inclusive
For a requirement that accepts integer ages from 18 through 120 inclusive, the equivalence partitions are below 18 (invalid), 18–120 (valid), and above 120 (invalid). Equivalence partitioning suggests representatives from each class.
The ordered edges are 18 and 120. A two-value set using the boundary and the outside neighbor is 17, 18, 120, and 121. A three-value set using below, on, and above each edge is 17, 18, 19, 119, 120, and 121. Confirm inclusivity and the smallest meaningful increment from the actual requirement; this integer example does not automatically apply to domains with different ordering or precision.
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Decision tables: make business rules explicit
When outcomes depend on combinations of conditions, represent each relevant combination and its resulting action as a rule. This makes gaps visible: a condition may have been tested alone while an interaction with another condition remains unexamined. ASTQB’s Foundation Level page, presenting ISTQB syllabus material, states: “Decision tables are used for testing the implementation of requirements that specify how different combinations of conditions result in different outcomes.”
Keep the table tied to the requirement. Identify the conditions, enumerate relevant combinations, and map each rule to an expected action or outcome. If you simplify rules, verify that the simplification does not discard combinations needed for the coverage goal.
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State-transition testing: include history and events
When the same event can produce different behavior depending on what happened before, model the meaningful states and the events—or guarded events—that move the system between them. Derive cases from state changes and paths, then select state, transition, or path coverage according to risk and the behavior that matters. Include preconditions so a case begins in the state its expected result assumes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to combine techniques without leaving gaps
Choose more than one technique when the feature contains more than one kind of structure. A bounded form field may need partitions and boundary tests; an eligibility rule may need a decision table as well as boundary checks on numeric inputs; a workflow may need transition tests for its lifecycle and partitions for the data accepted at each step.
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- Use partitions to reduce a broad input space to meaningful classes.
- Use boundaries when limits or adjacent ordered values could be treated differently.
- Use decision tables when conditions interact to determine outcomes.
- Use state models when the current state or event history changes behavior.
- Consider structural or experience-based testing when code structure or practitioner knowledge points to risks the requirement model does not expose.
The wider testing toolkit also includes black-box, white-box, experience-based, and collaboration-based approaches. The appropriate mix depends on the system, risks, requirements, standards, and practitioner skill; the four techniques above do not rank every possible approach.
Keep the test cases usable
For every derived case, capture the setup and expected result precisely enough that someone can determine whether it passed. Link it to the requirement or model element it covers. When reviewing a set, ask whether invalid inputs, important condition combinations, reachable states, and relevant neighboring boundary values are represented. Coverage should reflect risk and intended behavior rather than an unexamined count of cases.
Further reading
For deeper reading specifically on model-based testing and how models relate to classic techniques, see Model-Based Testing Essentials: Guide to the ISTQB Certified Model-Based Tester.
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