Complex longitudinal and cross-sectional research designs may involve subsegments of a sample being asked different questions or only some of the questions in a given study. In the case of attitude items, for example, the number of objects under study and the number of attributes of each object may combine to make a questionnaire burdensome for a subject, especially when remeasurement is involved. In this case a particular subject may be asked to evaluate only a subset of attributes or objects. In longitudinal studies (particularly panel designs), systematic changes in knowledge may be incorporated into a question structure which changes over time with questions added and others dropped as useful information accrues or as new issues become relevant.