A Data Dictionary, or data codebook, defines and describes the elements of a dataset so that it can be understood and used at a later date. Data documentation can be maintained in a variety of forms. Explore additional Documentation & Metadata practices.

What is a Data Dictionary?

A data dictionary is a document that outlines the structure, content, and variable definitions for a data collection. They are frequently formatted as a table or spreadsheet. A data dictionary is a critical tool for data analysis and reproducibility.

A data dictionary should include an overall description of the data:

  • Variable name
  • Variable meaning
  • Variable units
  • Variable format
  • Variable relationships
  • Variable coding values and meanings
  • Null value indicator
  • Anything else someone needs to know to better understand the data

The term codebook is often used interchangeably with data dictionary, though the data dictionary can contain more information about the structure of a database. For example, in REDCap the data dictionary is a CSV file containing information on the variables and the structure of the REDCap database, while the codebook is a human readable document that provides information on each data element. 

It is useful to create a data dictionary to describe your tabular data and accompany deposit in a repository so that others can interpret and reuse your data.

  • If the data you are describing is tabular, the description could be in a tabular form as well. In most cases, you should create your dictionary as a plain text file.
  • A shared data dictionary facilitates standardization by documenting common data structures and providing the precise vocabulary for specific data elements. Shared dictionaries ensure that the meaning, relevance, and quality of data elements are the same for all users
  • Adopt and cite a data standard to guide data collection and definitions. Utilize naming conventions to further ensure standardization.
  • As new data elements are identified, add them to the data dictionary, and maintain an updated dictionary throughout the project lifecycle. 

Data Dictionary Resources