Boundary Value
A boundary value is a value located at the edge of an input or output range where defects are more likely to occur.
“A minimum or maximum value of an ordered equivalence partition.”
(Definition reproduced from the ISTQB Glossary. Copyright belongs to ISTQB.)
View the complete ISTQB GlossaryWhat is Boundary Value?
Boundary values represent the minimum, maximum, or values immediately adjacent to defined limits. These values are important because software often fails at boundaries.
Edge conditions: Defects frequently occur at the limits of valid and invalid ranges.
Relationship to BVA: Boundary values are the basis for Boundary Value Analysis test cases.
Testing benefit: Focusing on boundaries increases the likelihood of detecting defects efficiently.
Visual Example
Typical boundary layout (valid range in the middle)
Real World Example
A banking team is updating the daily transfer limit rules for personal accounts, including the exact minimum and maximum amounts a customer may send.
The risky part is not the middle of the range; it is the values sitting at, just below, and just above each limit. That is where Boundary value becomes practical instead of theoretical.
The tester turns the rule into focused test data, checks the valid edges, and deliberately tries values just outside the allowed range to expose off-by-one and validation defects.
Because the team understands Boundary value, they catch limit-handling defects before release instead of discovering them through failed customer transfers.
Practice Questions
Question 1
Which set BEST exercises boundaries for 1–5 inclusive?
Question 2
Why target boundary values?
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