Data Documentation Initiative (DDI-Codebook, DDI-C) is a more light-weight version of DDI-Lifecycle, intended primarily to document simple survey data. Originally DTD-based, DDI-C is now available as an XML Schema. The freely available international DDI standards describe data that result from observational methods in the social, behavioral, economic, and health sciences. DDI is used to document data in over 80 countries of the world.