How to test an interval within boundary values?

Boundary value testing when checking an interval involves testing the system's behavior at the edges of the allowable values and near them.

• Allowable interval: [Minimum value, Maximum value]
• Test points:
  • Minimum value
  • Minimum value + 1 (or the smallest possible increment)
  • Maximum value
  • Maximum value - 1 (or the smallest possible decrement)
  • A value less than the minimum (outside the interval)
  • A value greater than the maximum (outside the interval)
  • One or more values inside the interval (to check "normal" behavior)

Example scenarios:

Consider an age input field with an allowable interval from 1 to 120.

Test case	Input value	Expected result	Comment
Min boundary	1	Successful validation	Allowable minimum
Near min	2	Successful validation	Allowable
Max boundary	120	Successful validation	Allowable maximum
Near max	119	Successful validation	Allowable
Outside interval	0	Error message	Less than minimum
Outside interval	121	Error message	Greater than maximum
Inside interval	50	Successful validation	Typical value

This approach effectively identifies defects related to improper handling of boundary conditions, which are often sources of errors.