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National Science Foundation National Center for Science and Engineering Statistics
Trends in Interdisciplinary Dissertation Research: An Analysis of the Survey of Earned Doctorates

Combinations of Fields Reported in Interdisciplinary Dissertations



 

Next we examined which types of fields tend to be associated with each other in the reporting of interdisciplinary dissertations. We used the classifications provided with the Survey of Earned Doctorates as a guide to classify each secondary field of research by how closely related it is to the primary field of dissertation research. If the secondary field is in the same knowledge domain as the primary field (e.g., cellular biology combined with molecular biology), then the two disciplines are considered to be "closely" related. Alternatively, if the two fields are from two different knowledge domains (e.g., biology combined with sociology) they are considered to be "distantly" related.[5]

Sixty-four percent of interdisciplinary doctorate recipients from 2001–08 reported a secondary field that was closely related to their primary dissertation field. This means that only about 36% reported a distantly related secondary field. Thus a majority of respondents who conducted interdisciplinary dissertation research reported two similar fields of research. Table 4 presents the percentage of dissertations with two distantly related fields or two closely related fields, broken down by year.

TABLE 4. Interdisciplinary dissertations, by year and whether the secondary field is closely or distantly related: 2001–08
(Percent)
Year Closely related
secondary field
Distantly related
secondary field
Total 63.6 36.4
2001 68.8 31.2
2002 68.1 31.9
2003 68.8 31.2
2004 61.6 38.4
2005 60.9 39.1
2006 60.6 39.4
2007 61.1 38.9
2008 61.8 38.2
SOURCE: National Science Foundation/National Center for Science and Engineering Statistics, Survey of Earned Doctorates.

  Table 4 Source Data: Excel file

This table shows that the percentage of dissertations in which the secondary field was distantly related to the primary field increased in 2004. From 2001–03 the rate was approximately 31%–32%, but from 2004 onward the rate was 38%–39%. This is notable because the question wording changed in 2004 to include the term "interdisciplinary." Earlier in this report (see table 1), we noted that the overall rate of interdisciplinary dissertations did not seem to be affected by the question wording change. But in this case the question wording may have made a difference; the percentages in table 4 suggest that this methodological change might explain, at least in part, the sudden increase in respondents who reported distantly related fields. It seems as though the inclusion of the term "interdisciplinary" in the question after 2004 incited more people to report distantly related fields of dissertation research.

Next we examined whether the proportion who reported a closely or distantly related secondary dissertation field varied by primary dissertation research area. Table 5 demonstrates that the rate at which respondents reported a distantly related secondary research field varied dramatically by the knowledge domain of the primary research field. Figure 2 Figure. further illustrates this variation; it graphs the proportion of interdisciplinary dissertations using distantly related secondary fields by knowledge domain, ordered from highest to lowest. The values range from 18.7% (life sciences) to 87.7% (computer and information sciences).

TABLE 5. Closely and distantly related secondary research fields, by knowledge domain of primary field: 2001–08
(Percent)
Primary field Closely related
secondary field
Distantly related
secondary field
Total 63.6 36.4
Business management and administration 47.7 52.3
Communications 18.3 81.7
Computer and information sciences 12.3 87.7
Education 62.2 37.9
Engineering 59.8 40.2
Humanities 69.7 30.3
Life sciences 81.3 18.7
Mathematics 37.5 62.5
Physical sciences 57.3 42.7
Psychology 55.1 44.9
Social sciences 49.6 50.4
SOURCE: National Science Foundation/National Center for Science and Engineering Statistics, Survey of Earned Doctorates.

  Table 5 Source Data: Excel file

The meaning of these numbers should be interpreted with caution, however, because they appear to be highly negatively correlated with the number of fields within each of these knowledge domains.[6] For example, the high proportion of distantly related fields for computer and information sciences is partially due to the fact that this field category contains only three fields, so there is less likelihood of a person being able to list two fields within this category than among the other categories. Similarly, the communications category, which has the second highest rate of 81.7%, contains only six fields, which may also limit the likelihood of an individual reporting two fields within this category. The life sciences category, which has the lowest rate of cross-category combinations (18.7%), is the category with the largest number of fields to select (n = 70), thereby increasing the likelihood that a person's secondary field is within the same category as their first. Figure 3 Figure. illustrates the negative relationship between field category size and the percentage of distantly related secondary research fields. There is almost a perfect, negative relationship between the number of fields in a knowledge category and the percentage of dissertations within that category which list a secondary field in another knowledge category.

Over the years, all of the field categories exhibited some increase in the percentage of secondary fields that are distantly related. Thus, as discussed earlier, over time there was a slightly greater tendency for individuals to combine two seemingly unrelated fields in their dissertation research. However, the knowledge domains with the highest increase in divergent fields were not necessarily the fields that displayed the greatest increases in interdisciplinary research in general.

Notes

[5] The doctorate earners were not included in these analyses if they reported a primary field that is classified in the "other fields not elsewhere classified" (miscellaneous) category. This miscellaneous category contains a variety of fields that are not related to each other in any meaningful way, so it does not make sense to compare them, as a group, to other fields.

[6] Logistic regression analyses confirm that controlling for the size of the field category reduces the strength of the relationship between field category and percentage of secondary fields that are distantly related to the first field.

 
Trends in Interdisciplinary Dissertation Research: An Analysis of the Survey of Earned Doctorates
Working Paper | NCSES 12-200 | April 2012