From the very beginning of a longitudinal survey, management of metadata is crucial. Structuring metadata in a standardized form greatly increases its utility, making metadata driven processes possible. Software using metadata may generate survey instruments and samples, administer an online survey, and produce tables and graphs of summary statistics. Good metadata in structured form makes higher quality documentation more accessible and can aid in retrieval of selected data. Quality metadata in structured form should facilitate replication of data collection and analyses across waves of a longitudinal survey. Replication of all or part of a survey in other places or times will depend on accessible metadata.
Longitudinal surveys have special needs for good metadata management, in part due to the length of time over which they exist. Procedures may evolve, staff may change, relevant external events may occur. Recording linkages and comparison between versions of questions and response sets will aid in harmonization. Knowledge of events which occur through the life of the study may be crucial in interpretation of the data.
DDI 3 (http://www.ddialliance.org/) is a metadata specification designed for the whole data lifecycle, from initial concept to archive and reuse. It includes features relevant to longitudinal surveys such as metadata repositories, the capacity to record lifecycle events and comparisons between metadata elements. The rich set of elements in DDI 3 allows data collection, tabulation, and display to be driven by metadata.