The Data Documentation Initiative (also known as DDI or DDI Metadata) is an international standard for describing surveys, questionnaires, statistical data files, and social sciences study-level information. This information is described as metadata by the standard.
The 2010 Census is one of the most recognizable research questionnaires in the United States. The US Census is advertised on talk shows, in the media, and is mailed to every current home address. This familiarity makes the 2010 Census a great questionnaire to demonstrate some features of the DDI 3 metadata standard and its Data Collection component.
The Supplemental Security Income (SSI) Public-Use Microdata File contains an extract of data fields from the Social Security Administration's (SSA) Supplemental Security Record file. This dataset is used to demonstrate how to document variables and datasets in DDI 3.