<p>Over the course of a multi-year (longitudinal) study, as social sciences researchers gain an improved understanding of the study's domain and subjects, it may be necessary to make incremental changes to the study's data collection procedures. Drag, Drop, Merge (DDM) is a set of tools for working with data collected under evolving methodologies, allowing study designers to flag differences between study iterations, and allowing researchers to quickly select the subset of the data that is appropriate for their analysis.</p>