2.2Boundary Value Analysis (BVA)

Test at the edges of input ranges, where bugs cluster. Off-by-one errors are the most common numerical bug in software, and BVA is specifically designed to catch them.

The Principle

For any input with a valid range, test:

• The value just below the lower boundary (invalid)
• The lower boundary itself (valid)
• The value just above the lower boundary (valid)
• The value just below the upper boundary (valid)
• The upper boundary itself (valid)
• The value just above the upper boundary (invalid)

Example: Age Field (accepts 18-65)

Test Value	Expected	Rationale
17	Rejected	Just below minimum
18	Accepted	Lower boundary
19	Accepted	Just above minimum
64	Accepted	Just below maximum
65	Accepted	Upper boundary
66	Rejected	Just above maximum

Additional Boundaries to Consider

• Zero: Does the field accept 0? What about negative numbers?
• Empty string vs null: Different things in most systems
• Maximum length: For text fields, what happens at max length, max + 1?
• Data type limits: Integer overflow (2,147,483,647 for 32-bit signed int)
• Date boundaries: End of month (28/29/30/31), year boundaries, leap years

BVA in Practice

GIVEN a registration form with age field (valid range: 18-65)
WHEN  the user enters age = 17 and submits
THEN  the form shows validation error "Age must be between 18 and 65"

GIVEN a registration form with age field (valid range: 18-65)
WHEN  the user enters age = 18 and submits
THEN  the form accepts the input and proceeds to the next step