Let n₁ be the number of requirements for which all reviewers have identical interpretations and n₂ be the total number of requirements. Which of the following is used to determine the specificity of the requirements?

Explanation

Pressman defines specificity (absence of ambiguity) as Q₁ = n₁ / n₂, the fraction of requirements that every reviewer interpreted in the same way. Since n₁ ≤ n₂, this ratio lies between 0 and 1, and the closer it is to 1 the less ambiguous the specification — a property a sensible metric must have. Option 2 inverts the ratio and would exceed 1, while options 3 and 4 are a raw sum and a raw difference that depend on the size of the specification rather than on the proportion of unambiguous requirements, so none of them can serve as a normalised quality measure.