DDI Common Core

 A slightly modified version of this document is expected to be published as an ISO standard (ISO 25955) 

The DDI Common Core defines and specifies the set of elements that sit at the core of all DDI products. It defines the Variable Cascade, Unit Cascade, and Value Domains, within the Data Lifecycle. 

DDI first began in 1995 to build a framework for describing a codebook, the document describing the variables, questions, code lists, classifications, methodologies, usage guidance used to collect, organize, and delimit the data in research studies. Over the years DDI has grown to provide three major specifications as well as a number of more focused products that support the functionality of these specifications as well as provide generalized tools for the broader metadata community. 

Each specification in DDI expands upon this common core by adding detailed content relating to areas within the lifecycle covering data sources, methodologies, data capture, acquisition,  dissemination, preservation, physical storage structures, processing activities, analysis, summary content, and related resources. The following table (pg 10, DDI Common Core, 2025) summarizes the areas of specialization of the current DDI specifications. (To view individual product specifications, click on product name in the table.)