In questionnaire design, there is often the requirement to ask questions about brands, topics or products which must be selected according to criteria such as: answers to previous questions, degree of quota completion etc. Sometimes there is a need to randomly select only one out of several brands which meet the same criteria.
Design provides a special tool which makes it easy to specify succinctly the selection rules for even the most complex situations. The tool is accessed by specifying a $S mask in a question. This mask contains a list of all the possible brands or topics out of which the selection(s) must be made. We call this a Selection question.