[Excerpt] According to the Office of Personnel Management (OPM), the federal workforce is composed of an estimated 2.1 million civilian workers. Several federal agencies collect, compile, and publish statistics about this workforce. Sources may vary in their totals due to differences in the methods used to compile these statistics. For example, some sources rely on “head counts” of employees (OPM), some on total hours worked (such as the Office of Management and Budget [OMB]), some on surveys of employing agencies, and others on self-identification by workers surveyed in their homes. In addition, federal civilian employee databases may exclude particular departments, agencies, or branches of government. Some may also account for temporary or seasonal employees (such as those employed by the U.S. Census) depending on the time of year the statistics are generated. This report describes these sources and identifies key differences in methodologies, including data collection used by OMB and OPM. Understanding these sources and their differences will facilitate selecting appropriate data for specific purposes.