Black Box · Specification-Based

Decision Table Testing

Map every combination of conditions to their expected actions in a table. Each column is a test case. This technique is the gold standard for testing complex business rules.

Junior Senior ISTQB CTFL v4.0 — 4.2.3

What it is

A decision table is a matrix that systematically covers all combinations of input conditions and shows what action should result from each combination. It forces you to think about every scenario — including the ones you'd otherwise miss.

It's most useful when: multiple conditions interact, business rules are complex, or you need to prove exhaustive coverage to stakeholders.

Table structure

Conditions (rows, top half) — the inputs or rules that can be true or false.
Actions (rows, bottom half) — what the system does in response.
Rules (columns) — each column is one combination of conditions. Each column = one test case.

For N binary conditions, the full table has 2^N columns. 3 conditions = 8 test cases. Many can be merged when the action is identical regardless of one condition's value.

Worked example

A discount system: users get a discount if they are a member AND their order total is > $100. Members always get free shipping; non-members only get free shipping on orders > $100.

Discount & shipping decision table

Condition / Action	Rule 1	Rule 2	Rule 3	Rule 4
Is member?	Y	Y	N	N
Order > $100?	Y	N	Y	N
Apply discount?	Yes	No	No	No
Free shipping?	Yes	Yes	Yes	No

Each column (Rule 1–4) maps directly to a test case with defined inputs and expected outcomes. No ambiguity, no missed scenarios.

Reducing the table

When an action is the same regardless of one condition's value, you can collapse columns using a "don't care" value (often written as — or *). This reduces test count while maintaining coverage.

In the example above, free shipping for members applies regardless of order total — Rules 1 and 2 could be merged for the free-shipping action.

ISTQB mapping

ISTQB CTFL v4.0 reference

Ref	Topic	Level
4.2.3	Decision Table Testing	CTFL Foundation
FL-4.2.3 K3	Apply decision table testing to derive test cases	Foundation LO
CTAL-TA 3.2.3	Advanced decision tables, minimisation, cause-effect	Advanced / Senior

When to use it

When there are 2–5 conditions with interactions that matter
When business logic is defined as "if A and B but not C, then..."
When you need to show a stakeholder you've covered all scenarios
As a precursor to test case writing — the table is your test design

Avoid for simple cases: If there's only one condition, use EP/BVA instead. Decision tables shine with 2+ interacting conditions.

Practice this technique: Try Junior Practice 09 — Checkbox & radio logic, Junior Practice 04 — NZ checkout form.

NZ example — GST pricing rules

New Zealand GST (Goods and Services Tax) is 15%. An e-commerce checkout must apply GST correctly based on two conditions: whether the customer is GST-registered (they can claim it back) and whether the product is GST-exempt (some items like financial services are exempt).

NZ GST decision table — 4 rules, 4 test cases

Condition / Action	Rule 1	Rule 2	Rule 3	Rule 4
Customer is GST-registered?	Y	Y	N	N
Product is GST-exempt?	N	Y	N	Y
Show price	Ex-GST	GST-exempt	GST-inclusive	GST-exempt
Add GST line item?	Yes	No	No	No

Each rule = one test case. The decision table ensures all four combinations are covered. A common bug is that GST-registered customers get charged GST on exempt products (Rule 2 missing or wrong).

Try it yourself

NZ online store — free shipping decision table

A NZ online store gives free shipping based on two conditions: (A) customer has an active loyalty card, (B) order total is $75 or more. The condition rows are filled in for you — complete the action rows by selecting Yes or No for each rule.

Condition / Action	Rule 1	Rule 2	Rule 3	Rule 4
Loyalty card?	Y	Y	N	N
Order ≥ $75?	Y	N	Y	N
Free shipping?
Loyalty discount?

Completed decision table:

Condition / Action	Rule 1	Rule 2	Rule 3	Rule 4
Loyalty card?	Y	Y	N	N
Order ≥ $75?	Y	N	Y	N
Free shipping?	Yes	Yes	Yes	No
Loyalty discount?	Yes	Yes	No	No

Free shipping: loyalty card OR order ≥ $75 (either condition is sufficient).
Loyalty discount: loyalty card only (regardless of order total).

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