DDI-Codebook Development History
DDI Codebook was the first version of the DDI specification to be published (Version 1 was released in 2000). DDI Codebook 2.0 was released in 2003, with Version 2.1 following two years later, adding coverage of aggregate data and geography. The canonical expression of DDI 1.* through 2.1 is a Document Type Definition (DTD), although XML Schema versions have been created.
The most recent version in the DDI Codebook line is 2.5, published January 29, 2014. This version is an XML Schema. It incorporates new substantive elements requested by the community and is designed to make it easier to migrate documents to DDI Lifecycle for those interested in doing so. DDI Codebook 2.5 is backward compatible with DDI 2.1.