In a survey that requests participants to select their age group from the following: 20-30, 30-40, 40-50, and 50+, which key principle of designing closed-ended questionnaire items is being violated?

Explanation

The issue lies in the overlapping age ranges, such as 30 appearing in both 20-30 and 30-40 categories. This violates the rule of having mutually exclusive options, where each response should fit into only one category without ambiguity.