by Jaquelina C. Falkenheim
Interdisciplinary research has been recognized as a key component of advances in science and engineering (National Academy of Sciences 2005), yet published data on interdisciplinarity are scarce. This InfoBrief examines one indicator of interdisciplinary research: the number of respondents to the Survey of Earned Doctorates (SED) who report two or more dissertation research fields.
Based on this indicator of interdisciplinarity, over the period from 2001 to 2008, the share of doctorate recipients reporting interdisciplinary dissertation research fluctuated between 24% and 30%. Most interdisciplinary dissertation research was conducted in the same knowledge domain (science, engineering, or non-science and engineering fields) as the primary research. At the broad-field level, doctorate holders with dissertations in biological sciences were the most likely (81%) to list secondary dissertation research in that same broad field, and those with primary dissertations in computer science and mathematics were the least likely to do so (11% and 29%, respectively).
What Constitutes Interdisciplinary Research?
In this InfoBrief a dissertation is considered to be interdisciplinary if the SED respondent provides a valid response to (1) primary field of dissertation research and (2) at least a second dissertation field (third and fourth field of dissertation if applicable). The analysis uses the taxonomy of fields provided to respondents at the end of the SED questionnaire.
The SED has collected information on doctorate recipients' primary fields of dissertation research since 1958, and it began collecting data on secondary fields of dissertation research in 2001. In 2004, the survey began explicit data collection on interdisciplinary dissertations. This InfoBrief presents data from 2001 to 2003 to establish a baseline level of doctorate recipients who reported having secondary dissertation fields. The analysis focuses on data collected under the current, explicit wording (2004–08), which provides a consistent definition of interdisciplinary research.
Interdisciplinarity at the Dissertation Level
Between 2001 and 2003, the proportion of respondents reporting secondary dissertation research fields increased slightly, from 24% to 28% of all doctorate recipients who reported their primary dissertation fields (table 1). In 2004, with the explicit question wording, 28% of the doctorate recipients who responded to this question reported interdisciplinary dissertation research. This proportion remained relatively stable through 2008.
Table 1 Source Data: Excel file
Few respondents reported more than two dissertation research fields between 2004 and 2008. Of the 44,032 respondents in 2008 who reported the primary field of their dissertation research, 27% indicated a second field, 1% a third field, and less than 1% a fourth field. The percentage of respondents with three and four dissertation research fields did not change much over this 5-year period.
Doctorate recipients with primary dissertation research in agricultural sciences (39%), biological sciences (35%), and earth, atmospheric, and ocean sciences (35%) were the most likely to report more than one dissertation research field over this period, whereas computer sciences, mathematics, and psychology recipients were least likely to do so (20%, 21%, and 22%, respectively) (table 2).
Table 2 Source Data: Excel file
Recipients with multiple dissertation research fields in some S&E fields had high rates of non-S&E secondary topics: 37% of those in the social sciences, 29% in psychology, and 19% in computer sciences.
Respondents who reported primary dissertation fields in the sciences most frequently listed secondary research fields within the same broad fields in the sciences. However, this varied considerably by field of primary dissertation research, from the biological sciences (81%) to computer sciences (11%). About half of the doctorate recipients who reported primary dissertation research fields in earth, atmospheric and ocean sciences; physical sciences; psychology; or social sciences reported secondary dissertation research fields within the same broad fields; as did nearly 60% of those in engineering.
Biological sciences was the most frequently listed secondary S&E field across other sciences, except social sciences and engineering. Among those whose primary dissertation research was in agricultural sciences, almost 32% listed secondary research in biological sciences, a percentage almost identical to that of agricultural sciences doctorate holders who listed their secondary field as another agricultural science.
About 29% of mathematics and 11% of computer sciences doctorate holders reporting multiple dissertation research fields listed secondary fields within the same respective major fields. Dissertations with a computer sciences primary research field were most frequently combined with engineering (23%) or non-S&E (19%) secondary dissertation research fields. Dissertations with a mathematics primary research field were most frequently combined with biological sciences (25%) or engineering (12%).
Physical and biological sciences were the largest secondary fields for engineering doctorate holders.
Interdisciplinarity at the Institution Level
Examination of the proportion of all new doctorate holders who reported interdisciplinary dissertation research in each institution reveals a considerable range. By this measure, the five institutions with the highest degree of interdisciplinarity were Massachusetts Institute of Technology (MIT); Boston University; Indiana University-Bloomington; University of Georgia; and Johns Hopkins University.
However, the top five institutions in terms of the total number of respondents (900 or more) with interdisciplinary dissertations were the University of California at Berkeley; MIT; Pennsylvania State University; University of Michigan-Ann Arbor; University of Minnesota-Twin Cities. Except for MIT, all these institutions were also among the 10 largest in terms of the total number of doctorate recipients graduating between 2004 and 2008 (table 3).
Table 3 Source Data: Excel file
The indicator of interdisciplinarity presented here is rather rudimentary, and ranked data related to interdisciplinary dissertations by university should be interpreted with caution, especially where differences are small. Changes in response rates across institutions and time as well as different combinations of program offerings affect figures reported.
Data Sources and Limitations
In 2001, the SED began asking research doctorate recipients to list the name and number of the secondary field of their dissertation research, asking, "If you had a secondary field for your dissertation research, list the name and number." In 2004, this question was changed to "If your dissertation research was interdisciplinary, list the name and number of your secondary field." Respondents who did so could indicate a third and fourth field for dissertation research if applicable. Questionnaires for recent years are available at http://www.nsf.gov/statistics/question.cfm#1.
The SED data collection includes a few variables that are considered critical items (e.g., field of degree, sex) and warrant extensive followups with survey respondents. The dissertation research field question is not considered critical, but response rates have been high and stable over the period examined, ranging from 90% to 92% in the 2001–08 period (table 1).
In this InfoBrief, percentages by field are based on the National Science Foundation's Science Resources Statistics (SRS) taxonomy of fields. Use of a different taxonomy may produce different results.
 Jaquelina C. Falkenheim, Science and Engineering Indicators Program, Division of Science Resources Statistics, National Science Foundation, 4201 Wilson Boulevard, Suite 965, Arlington, VA 22230 (firstname.lastname@example.org; 703-292-7798).
 The SED taxonomy is reviewed and updated every 3 years by analyzing the verbatim answers that respondents provide in the "other" category when their field of study does not fit in the existing taxonomy. Emerging fields are eligible for addition to the taxonomy if 10 or more doctoral degrees were awarded in the field by three or more universities in three consecutive years (10-3-3 rule). Some fields in the SED taxonomy may be considered intrinsically interdisciplinary, for instance, biochemistry or neuroscience. However, in this InfoBrief these examples are each regarded as a single field because both have met the 10-3-3 rule and have been incorporated into the SED taxonomy—biochemistry in 1958 and neuroscience in 1982. Appendix E in the documentation of the Doctorate Records File, available by request, includes information regarding the dates when fields were incorporated in the SED taxonomy.
National Academy of Sciences. 2005. Facilitating Interdisciplinary Research. National Academies of Science, National Academy of Engineering, and Institute of Medicine of the National Academies. Washington DC: The National Academies Press.