Negative statement
A negative form expresses what the system must not do. It uses words like “shall not,” “must not,” “no,” or “never.” Requirements written this way describe undesired behavior instead of clearly stating what the system should do. Avoid using negative forms in requirements. They can be ambiguous, harder to validate, and more likely to be misunderstood during implementation and testing. Instead, write requirements that describe expected behavior in a clear, positive way.
| Category | Description |
|---|---|
| Rule mapping (INCOSE) | R16 – USE OF “NOT” |
| Default weight | 15 |
| Terms to avoid | "not", "no", "none" |
Example 1: Implied expectations
Unclear Requirement: On pressing Enter, the user shall not move to the next page.
Issue: The requirement does not specify the intended behavior.
Clear Requirement: On pressing Enter, the system shall display an error message.
Example 2: Difficult to verify
Unclear Requirement: The system shall not allow unauthorized access.
Issue: It's hard to test for the absence of unauthorized access.
Clear Requirement: The system shall provide access only to authorized users.
Example 3: Risk of duplication
Unclear Requirement: The interface shall not show correct results in red font. The interface shall not show correct results in blue font.
Issue: Negative wording requires multiple statements to cover all undesired outcomes.
Clear Requirement: The interface shall show correct results in green font.