Robust and secure programming: Boundary data would be for example:

The idea

Boundary data is a category of test data chosen to sit exactly on, and immediately either side of, the limits of an allowed range. The statement is an introduction to examples, so the skill is recognising which values count as boundary data for a given limit. Take an allowed range and identify its lowest and highest permitted values; those are the boundaries. Then add the values just outside each boundary, because programs often fail there through off-by-one errors in comparisons. For example, if a program accepts ages from 16 to 18 inclusive, the boundary values are 15, 16, 18 and 19. Values such as 17 sit inside the range and are normal data, not boundary data. When answering, name the limits first, then list the values on and just beyond each limit, and say why each is chosen: to test that the program accepts valid edge values and rejects invalid ones just outside.

What earns the marks

• Defines boundary data as test data at the exact limits of an allowed range and immediately outside those limits.
• Identifies the lower and upper permitted values of a stated range as the boundaries.
• Includes values one step below the lower limit and one step above the upper limit.
• Explains that boundary values are chosen because comparison errors commonly occur at the edges of a range.
• Distinguishes boundary data from normal data that lies well inside the range.

Common mistakes and how to put them right

• Treating only the values inside the range as boundary data.

  Put it right: Boundary data includes the values immediately outside each limit as well as the limits themselves.

• Confusing boundary data with erroneous data or invalid data generally.

  Put it right: Boundary data is a deliberate test-data category targeting the edges of a valid range.

• Listing every value in a small range as boundary data.

  Put it right: Only the limits and the values just beyond them are boundary values, while interior values are normal data.

Exam tips

• State the allowed range explicitly before listing boundary values, so the examiner can see the limits you are working from.
• For each boundary value, say whether it should be accepted or rejected, linking it to the comparison in the program.
• Keep the step size consistent: move exactly one unit beyond each limit rather than choosing an arbitrary distant value.