If all the dictionaries in the world suddenly vanished, people would create as many different spellings and meanings for words as they could dream up—and disorder would overwhelm communications.
A data dictionary is an agreed-upon set of clearly and consistently defined elements, definitions, and attributes—and is indispensable to any information system. In the same way that standard English dictionaries help us use the English language effectively, data dictionaries help organizations maintain consistency in their information systems. Beyond data collection, database users and managers refer to a data dictionary to find out where specific data are located, whether they were reported correctly, how to use them appropriately, and what their values mean. Like an owner's manual, a data dictionary helps the data user understand and work with data.
Although many items in a data dictionary can be classified as metadata, data dictionaries and metadata systems are not identical. Data dictionaries generally contain only some of the metadata necessary for understanding and navigating data elements and databases and, thus, contain only a subset of the metadata found in a robust metadata system. Metadata systems, on the other hand, generally include the entire range of items used for data system management and analysis, including features for sorting, searching, organizing, and connecting data and metadata (see exhibit 2.3).