Construction of a single global scale for multi-item assessments of the same variable.

Complex health-related concepts are commonly measured by means of multi-item scales or questionnaires. As the item scales of an instrument cover different aspects of the same variable, a single global response value is required of the concept to be measured. In this paper, different approaches that take into account the non-metric properties of ordered categorical data when aggregating multi-item scales into global scaling were suggested. A single score of equally important items is easily defined by the median. When the multi-items are constructed to identify the most serious sign of a particular state, the maximum categorical level of the items could be an appropriate single score of the variable. By means of the joint response scheme over two aspects of the same variable, a global scale with an optional number of ordered categories could be defined. Some of the multi-item scales of the Short-Form Health Survey (SF-36) and Euro-QoL were used for illustrations to the proposed approaches.