NCES has changed the types of institution reported over this time series. From 1995-1998, NCES reported two types of research or doctoral universities:
- Research universities: Committed to graduate education through the doctorate, give high priority to research, and receive more than $15.5 million in annual federal funds research.
- Doctoral universities: Offer a full range of baccalaureate programs and are committed to education through the doctorate. They award at least 40 doctoral degrees annually in five or more disciplines.
From 2000-2006, NCES reported two types of research or doctoral universities:
- Doctoral, extensive universities: Committed to graduate education through the doctorate, and award 50 or more doctor's degrees per year across at least 15 disciplines.
- Doctoral, intensive universities: Committed to education through the doctorate and award at least 10 doctor's degrees per year across 3 or more disciplines or at least 20 doctor's degrees overall.
Beginning in 2007, NCES began reporting three research or doctoral categories:
- Research universities (very high level): Research universities with a very high level of research activity.
- Research universities (high level): Research universities with a high level of research activity.
- Doctoral, research universities: Institutions that award at least 20 doctor's degrees per year, but did not have a high level of research activity.
In addition to the research or doctoral categories, NCES provides these definitions for the remaining categories shown:
- Masters: Award at least 50 master's degrees per year.
- Baccalaureate: Primarily emphasize undergraduate education.
- Special-focus: Award degrees primarily in single fields of study, such as medicine, business, fine arts, theology, and engineering. This final category includes some institutions that have 4-year programs, but have not reported sufficient data to identify program category. It also includes institutions classified as 4-year under the IPEDS system, which had been classified as 2-year in the Carnegie classification system because they primarily award associate's degrees.
Because research and doctoral category definitions have changed over time, this database maintains the original NCES definitions. Therefore, data are not reported in all years:
- The categories Research university and Doctoral university are reported only for 1995-1998
- The categories Doctoral, extensive universities and Doctoral, intensive universities are reported only for 2000-2006. (1999 data are not available.)
- The categories Research universities (very high level), Research universities (high level), and Doctoral, research universities are reported only for 2007-present.
In each year, however, the sum total of research and doctoral universities are reported.