There are a wide variety of doctoral degrees awarded to students in a number of different categories in the United States.
The Doctor's degree-research/scholarship is defined as "A Ph.D. or other doctor's degree that requires advanced work beyond the master's level, including the preparation and defense of a dissertation based on original research, or the planning and execution of an original project demonstrating substantial artistic or scholarly achievement."
[5] For the purposes of this survey, a research doctorate is defined as "a doctoral degree that (1) requires completion of an original intellectual contribution in the form of a dissertation or an equivalent culminating project (e.g., musical composition) and (2) is not primarily intended as a degree for the practice of a profession.
[7] The Department of Education's 2008 Structure of US Education listed 24 frequently awarded research doctorates titles accepted by the National Science Foundation (NSF) as representing "degrees equivalent in content and level to the Ph.D".
Universities: Summary Report 2006 this was reduced to 18,[10] part of an ongoing program of assessment that saw the number of recognized research degrees reduced from the 52 recognized from 1994 (the earliest report archived online) to 1998, falling to 48 from 1999 to 2003 and to 24 in 2004.
[20] While research doctorates require "advanced work beyond the master's level, including the preparation and defense of a dissertation based on original research, or the planning and execution of an original project demonstrating substantial artistic or scholarly achievement",[21] professional doctorates must have a total time to degree (including prior study at bachelor's level) of at least six years, and provide "the knowledge and skills for the recognition, credential, or license required for professional practice".