Explain boundary value analysis and equivalence partitioning, and show how you would use them together on a field.

What the interviewer wants to hear

• Equivalence partitioning splits inputs into groups that should behave the same
• One value per partition is enough in theory
• Boundary value analysis tests values at and just outside the edges of partitions
• Defects cluster at edges because of mistakes like < instead of <=
• Concrete example with correct values

Model answer

Equivalence partitioning groups inputs that the system should treat the same way, so I test one value from each group instead of every value. Boundary value analysis adds tests at the edges of those groups, because developers often make off-by-one mistakes, like writing less than instead of less than or equal. Take a quantity field that allows 1 to 10 items. The partitions are below 1 (invalid), 1 to 10 (valid) and above 10 (invalid), so I might pick 0, 5 and 11. With two-value BVA I add the edges: 0, 1, 10 and 11. I also think about the input type, such as decimals, empty input, negative numbers and letters, which form extra invalid partitions. Together the two techniques give strong coverage with only a handful of tests.

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