Exploring Mutual Complementarity of Free-Text and
Controlled-Vocabulary Collection-Level Subject Metadata
in Large-Scale Digital Libraries: A Comparative Analysis
OKSANA L. ZAVALINA
College of Information, University of North Texas, Denton, Texas, USA
Oksana.Zavalina@unt.edu
ABSTRACT
Provision of high-quality subject metadata is crucial for organizing adequate subject access to
rich content aggregated by digital libraries. A number of large-scale digital libraries
worldwide are now generating subject metadata to describe not only individual objects but
entire digital collections as integral whole. However, little research to date has been
conducted to empirically evaluate the quality of this collection-level subject metadata. The
study presented in this paper compares free-text and controlled-vocabulary collection-level
subject metadata in three large-scale digital cultural heritage aggregations in the United States
and the European Union. As observed by this study, the emerging best practices in creating
rich collection-level subject metadata includes describing collection’s subject matter with
mutually-complementary values in controlled-vocabulary and free-text subject metadata
fields.
INTRODUCTION
Both cultural heritage institutions and funding agencies worldwide have invested
intensively in digitization projects. Large-scale digital libraries now bring together hundreds
of individual digital collections produced by these digitization projects.
Metadata — “structured data about an object that supports functions associated with
the designated object” (Greenberg, 2005, p. 1876) — is used in digital libraries to organize
information for effective retrieval via search and browse functions. Metadata is subdivided
into two distinct kinds: controlled-vocabulary metadata which draws values from formallymaintained list of terms, and free-text metadata which relies on natural language. The subject
metadata – “information concerning what the resource is about and what it is relevant for”
(Soergel, 2009) is crucial for providing subject access to information objects in digital
collections and aggregations. To help achieve optimal recall and precision, it is recommended
(e.g., ALCTS, 1999) to include Subject, Type, and Coverage elements in metadata records in
digital libraries to accommodate different subject-related facets: topic, place, time period,
language, etc.
Metadata that describes collections as integral whole has long been applied in archival
community. Many digital aggregations are now supplying collection-level metadata —
“metadata providing a high-level description of an aggregation of individual items”
(Macgregor, 2003, p. 248) — as means of providing context for the digital items harvested
from distributed collections. However, virtually no research to date has evaluated and
compared the collection-level metadata in digital aggregations.
In discussions of metadata, the terms “richness,” “detailed description,” “level of
description” or “quality” of metadata seem to be used interchangeably (e.g., Arms, 1998;
Duval, Hodgins, Sutton, & Weibel, 2002). The three most important metadata quality criteria
are accuracy, consistency, and completeness (Park, 2009; Park & Tosaka, 2010). Metadata
accuracy is measured as the degree to which the metadata values match characteristics of the
described object (e.g., Stvilia, Gasser, Twidale, & Smith, 2007). Metadata consistency is
further subdivided into semantic and structural consistency (Park, 2009). Semantic
consistency refers to an extent to which the same values or elements are used for representing
similar concepts, while structural consistency is evaluated as a degree to which the same
structure is followed in representing information in certain metadata elements (Bruce &
Hillmann, 2004). Metadata completeness is evaluated as an extent to which objects are
described using all applicable metadata elements to their full access capacity. Some of the
assessment criteria used to evaluate metadata completeness (Moen, Stewart, & McClure,
1998) include the number of metadata elements per record, practice of presenting blank (i.e.,
nonpopulated but displayed) metadata elements, utilization and selected characteristics of
mandatory and optional elements.
While evaluation of metadata in digital libraries, which has not yet become a common
practice, gains more and more importance to ensure metadata quality (Hillmann, 2008),
almost no research to date has attempted to evaluate collection-level metadata. Zavalina,
Palmer, Jackson, and Han (2008) started addressing this research gap by assessing collectionlevel metadata in the Digital Collections and Content registry of IMLS-funded digital
collections. However, because that study focused on a single digital library, generalizabilty of
its results is limited.
To produce more generalizable results, Zavalina (2011) study examined and
compared the free-text collection-level subject metadata (i.e., data values in Description
metadata element) across multiple digital libraries, and found that a variety of information
about a digital collection is included in free-text collection-level Description metadata
element. This includes both subject-specific (topical, geographic and temporal coverage, and
types/genres of objects in a digital collection) and non-subject-specific information: title,
size, provenance, collection development, copyright, audience, navigation and functionality,
language of items in a digital collection, frequency of additions, institutions that host a digital
collection or contribute to it, funding sources, item creators, importance, uniqueness, and
comprehensiveness of a digital collection. The study presented in this paper extends the
comparative analysis reported in Zavalina (2011) study by comparing the data values in freetext Description and four controlled-vocabulary subject metadata fields in three large-scale
digital libraries.
DATA COLLECTION AND ANALYSIS
Three large-scale cultural heritage digital libraries were selected for analysis:
American Memory1 developed by the United States Library of Congress, Opening History2
developed by the University of Illinois at Urbana-Champaign, and The European Library3
that aggregates digital collections created by the national libraries in the European Union.
Among these three digital libraries, only the Opening History displays its entire humanreadable collection-level metadata records. American Memory and The European Library
keep most of collection-level metadata (except for the Title and free-text Description
elements) behind the scenes to support search and faceted browse functions. For this study,
the XML files with complete collection metadata records were obtained from the developers
of The European Library and American Memory.
The systematic sample of collection-level metadata records in the three digital
libraries was analyzed: 39 records from American Memory, 33 records from Opening
1
http://memory.loc.gov.
http://imlsdcc.grainger.uiuc.edu/history.
3
http://www.theeuropeanlibrary.org.
2
2
History, and 27 records from The European Library. The resulting 99 collection-level
metadata records were subjected to detailed manual qualitative content analysis to determine
how the data values in different collection-level subject metadata elements within a record
relate to each other. These relations were categorized in three categories: one-way or twoway complementarity, and redundancy. The findings of the study are presented below.
FINDINGS
A significant proportion of collection metadata records in the sample included cases
of one-way complementarity, when information in one collection-level subject metadata
elements complemented information in one or more other metadata elements, by providing
additional details absent elsewhere. The highest occurrence of one-way complementarity
between collection-level subject metadata elements was observed in Opening History. In 76%
of collection metadata records analyzed in this study it was the free-text Description metadata
element that complemented information found in one or more of the controlled-vocabulary
subject metadata elements: Subjects, Geographic Coverage, Temporal Coverage, and
Objects.
As seen in the Figure 1, the free-text Description metadata element data values most
often (76% of records overall: 86% in American Memory, 76% in Opening History, and 70%
in The European Library) complemented topical information found in the Subjects element.
Representative examples include: “Spanish cartographer, … history, urbanism, public works
and agriculture from a strictly geographic point of view” in Description vs. “900 History and
geography, 911 Historical geography” in Subjects; “interior design, … homes of U.S.
presidents” in Description, with these topics not mentioned in Subjects; “early developments
in the National Park, … landscape and park facilities” in Description vs. “Great Basin, Social
studies, State history” in Subjects.
FIGURE 1. Mutual complementarity between collection-level subject metadata elements
Objects metadata element was the second most often complemented by object-typeor genre-specific information in Description field (49% overall: 70% in American Memory,
44% in The European Library, and 30% in Opening History). Representative examples
included: “uniform books, ego documents, photographs and sketches” in Description vs.
“images” in Objects; “digital pre-print originals and online publications” in Description while
Objects field was missing; “historical photographs, … portraits, … aerial shots” in
Description vs. “photographs/slides/negatives” in Objects; “rare books, government
documents, manuscripts, maps, musical scores, plays, films, and recordings” in Description
vs. “software, multimedia” in Objects.
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Data values in Temporal Coverage metadata element were also often complemented
by Description (46% overall: 67% in Opening History, 51% in American Memory, and 15%
in The European Library). Representative examples included: “16th century, 17th century, 18th
century, 19th century, 20th century” in Temporal Coverage vs. “Since the Eighty Years’ War”
in Description; “from 1895-1920s” in Description vs. “1850-1899, 1900-1929” in Temporal
Coverage field.
Geographic Coverage data values were complemented by Description metadata
element the least often (29% overall: 39% in Opening History, 33% in The European Library,
and 19% in American Memory). Representative examples included: “Hispanic America …
Spanish territories in America and Oceania” in Description vs. “Hispanic America” in
Geographic Coverage; “Hungary or the Central European region” in Description vs.
machine-readable “hu” in Geographic Coverage; “American states, the District of Columbia,
and London, England” in Description vs. “United States” in Geographic Coverage; “Baja
California, Mexico in an area south-east of Ensenada” vs. “Mexico (nation)” in Geographic
Coverage.
In addition, the cases of data values in free-text Description metadata element
complementing information contained in several controlled-vocabulary subject metadata
elements in the same collection-level metadata record were observed. In the example in
Figure 2, Description includes keywords that complement both Subjects and Objects with
topical information (“foodways, religious traditions, Native American culture, maritime
traditions, ethnic folk culture, material culture”), genre information (“children’s lore,”
“occupational lore,” “performances,” “interviews,” “surveys”), and occupational subject
information (“musicians, craftpersons, storytellers, folklife interpreters”), while also
specifying the dates encoded in Temporal Coverage field. In fact, in 22% of collection
metadata records in the sample (45% in Opening History, 11% in The European Library, and
10% in American Memory) Description field complemented two or more controlledvocabulary subject metadata fields.
FIGURE 2. Example of multiple complementarities
Data values in controlled-vocabulary subject metadata elements also complemented
data values in free-text Description (Figure 1). For example, in this same collection metadata
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record (Figure 4), Geographic Coverage provided spatial information absent in Description
(“United States (nation), Southern U.S. (general region), Florida (state)”), while Subjects
listed additional topics (e.g., “Architecture”) not covered by Description.
The Subjects metadata element was found to complement Description (Figure 1) the
most often – in 52% of collection metadata records overall (70% in Opening History, 60% in
The European Library, and 30% in American Memory). Representative examples included:
“860 Spanish and Portuguese literatures” in Subjects when this topic was not mentioned at all
in Description; “Tennessee Valley Authority, African Americans, forestry” in Subjects when
these topics were not mentioned at all in Description; 15 specific subject strings (e.g., “North
Carolina—African-Americans, North Carolina—Agriculture, North Carolina—Economics
and Business” in Subjects vs. much broader topical and spatial coverage in Description
(“North Carolina, … story of the Tar Heel State”).
The Temporal Coverage metadata element was found to complement Description in
43% of collection metadata records (72 % in The European Library and 67% in Opening
History, but only 3% in American Memory). Representative examples included: “1400s1699, 1700-1799, 1800-1849, 1850-1899, 1900-1929, 1930-1949, 1950-1969, 1970-1999,
2000 to present, Pre-1400” in Temporal Coverage when no time information was provided in
Description; “1783-1789” in Temporal Coverage when no time information was provided in
Description; “1200-1900” in Temporal Coverage vs. “European age of chivalry” in
Description.
The Geographic Coverage metadata element was found to complement Description
much more often than the Description complemented Geographic Coverage (Figure 1), or in
43% of collection metadata records overall (56% in The European Library, 55% in Opening
History, and 24% in American Memory). Representative examples included: “Poland,
Lithuania, Ukraine, Belarus” in Geographic Coverage vs. “Poland” in Description;
“Germany” in Geographic Coverage when no geographic information was provided at all in
Description; “Europe, Italy, Great Britain” in Geographic Coverage vs. “US and abroad” in
Description; “United States (nation), Midwest U.S. (general region), Illinois (state),
Randolph (county), Knox (county)” in Geographic Coverage vs. “Randolph County, Illinois”
in Description.
The Objects metadata element values also often complemented information found in
Description in two digital libraries — Opening History (52%) and American Memory (14%)
— for 23% of analyzed collection metadata records overall. No such trend was observed in
The European Library, which can be explained by inconsistent application of Objects
metadata element in this digital library: in 59% of collection metadata records in The
European Library sample the Objects metadata element was blank or missing, while in the
remaining 41% this field contained a broad single-word term (e.g., “images,” “maps”).
Representative examples of the Objects metadata element values complementing Description
included: “Film transparencies—Color, Cityscape photographs” in Objects vs. “photographs”
in Description; “Gelatin silver prints, Safety film negatives, Nitrate negatives” in Objects vs.
“original negatives and photographic prints” in Description; “books and pamphlets,
photographs / slides / negatives, newspapers, posters and broadsides, periodicals, prints and
drawings” in Objects vs. “manuscripts, photographs, ephemera and published materials” in
Description.
In addition, one-way complementarity between different controlled-vocabulary
metadata elements was also observed. In particular, geographical subdivisions (as in
“Japanese Americans—California—Manzanar”) and temporal qualifiers (as in “World War,
1914-1918”) in Subjects metadata element included information that complemented
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Temporal Coverage and Geographic Coverage values. In Opening History, Subjects
complemented Geographic Coverage in 12% of collection metadata records and Temporal
Coverage in 18% of the records in the sample.
The cases of two-way complementarity between the two collection-level subject
metadata elements were less numerous than cases of one-way complementarity. No cases of
two-way complementarity were observed between the two or more controlled-vocabulary
subject metadata elements. Two-way complementarity between the free-text (Description)
and controlled-vocabulary subject metadata elements, in contrast, occurred in 40% of
collection-level metadata records overall. Two-way complementarity was widespread in
Opening History (79% of records), but occurred less often in The European Library (41%)
and significantly less often in American Memory (8%). Most often two-way complemenarity
was observed between Description and Subjects elements (29% of collection metadata
records overall: 58% in Opening History, 30% in The European Library, and 5% in American
Memory). Two-way complementarity between Description and Temporal Coverage was
observed only in Opening History (39% in Opening History or 13% overall). Two-way
complementarity between Description and Geographic Coverage was observed in 11% of the
records overall: 24% in Opening History, 11% in The European Library, but in none of the
American Memory collection metadata records. The least overall two-way complementarity
was observed between Description and Objects metadata element (7% overall: 18% in
Opening History, 3% in American Memory and 0% in The European Library).
Representative examples of two-way complementarity included:





“letters” in Description vs. “autograph albums” in Subjects (taken together, the values
in two fields provide more comprehensive genre information).
“dance instruction manuals, anti-dance manuals, histories, treatises on etiquette” in
Description vs. “Ballroom dancing—United States” in Subjects (Subjects information
specifies Description information from “dance” to “ballroom dancing” and adds
geographic coverage information, while Description adds information on specific
aspects of dancing — “etiquette” — and genre of materials in collection not covered
by any other metadata field in this record).
“towns of Coal City, Braidwood, and Wilmington” in Description vs. “Illinois (state),
Grundy (county)” in Geographic Coverage (state and county information in
Geographic Coverage and town information in Description complement each other for
a more specific geographic representation).
“contemporary, … European age of chivalry, … prior to 1900” in Description vs.
“1200-1900” in Temporal Coverage (while Temporal Coverage specifies the lower
limit of the “prior to 1900” range of years — “1200” — and provides the time frame
for “European age of chivalry,” Description introduces another — “contemporary” —
time period not covered by Temporal Coverage).
“newspaper photographs” in Description vs. “photographs/slides/negatives, archival
finding aids” in Objects (Description specifies genre information in Objects from
general “photographs” to “newspaper photographs, while Objects adds another genre
not mentioned in Description — “archival finding aids”).
Among the digital libraries examined in this study, only The European Library had a
noticeable proportion (19%) of redundancy between the values in different collection-level
subject metadata elements. Very little redundancy was observed in the Opening History and
American Memory collection metadata records. Examples of redundancy include restating of
identical geographic information (e.g., “Estonia,” “Netherlands,” “Ljubljana” in both
Description and Geographic Coverage metadata element), temporal information (e.g.,
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“1763” in both Description and Temporal Coverage), and genre information (e.g.,
“photographs” in both Description and Subjects).
DISCUSSION AND CONCLUSIONS
The findings presented in this paper demonstrate high level of mutual
complementarity between free-text and controlled-vocabulary subject metadata in collectionlevel metadata records in three large-scale digital libraries that aggregate cultural heritage
digital collections: American Memory and Opening History in the United States of America,
and The European Library in Europe. Quite predictably, the data values in free-text
Description metadata element, due to its natural language values and higher length, often
complemented information in controlled-vocabulary subject metadata elements. However, it
was also observed in this study that data values in controlled-vocabulary subject metadata
elements, especially Geographic Coverage, complemented information encoded in
Description quite often. Both one-way complementarity and two-way complementarity was
observed, with little redundancy. Results of this study empirically demonstrate that more
detailed collection-level metadata records which include both free-text and controlledvocabulary subject metadata allow more fully representing intellectual content of information
objects and ultimately improving subject access for the users.
Completeness of metadata records — an extent to which objects are described using
all applicable metadata elements to their full access capacity — has long been emphasized as
one of the most important metadata quality criteria (e.g., Moen, Stewart, & McClure, 1998;
Bruce & Hillmann, 2004; Park, 2009; Park & Tosaka, 2010). Findings of the user studies
conducted both decades ago, with card catalogs and early computerized library catalogs,
(e.g., studies summarized by Krikelas, 1972), and more recently, with various online
information retrieval systems, (e.g., Wang & Soergel,1998; Drori, 2003; Crystal &
Greenberg, 2006; Smith-Yoshimura et al., 2010) demonstrate that users perceive both the
free-text subject metadata (e.g., the data values in MARC 5XX fields or Dublin Core
Description element) and controlled-vocabulary subject headings, such as the data values in
65X MARC fields or Dublin Core Subject and Coverage metadata elements, to be among
the most useful metadata elements to judge the relevance of retrieved documents Item-level
metadata records in digital libraries usually meet these user expectations by providing both
free-text and controlled-vocabulary subject metadata. However, most newly-created digital
libraries limit their collection-level metadata to free-text Title and Description elements for
various reasons: lack of resources needed to create detailed collection-level metadata records,
limitations introduced by the default settings in popular content management systems such as
DSpace, or even a belief that full-text indexing and keyword searching make controlledvocabulary subject metadata redundant. Lack of best practice guidelines for creation of
collection-level metadata arguably contributes to this situation.
Results of this study indicate that including mutually-complementary subject
information in free-text and controlled-vocabulary collection-level metadata elements is
already a common practice among some of the large-scale digital libraries, and possibly is
recognized by digital library developers as a benchmark in crafting rich collection-level
metadata. The findings of this study could be instrumental in developing best practice
recommendations for creating collection-level metadata, including subject metadata, which
are not currently available. These guidelines can be incorporated in the next edition of the
Framework of Guidance for Building Good Digital Collections (NISO Framework Working
Group, 2007) and/or the Guidelines for Digital Libraries that are currently being prepared by
the IFLA working group jointly with the World Digital Library Project.
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This exploratory study focused on collection-level subject metadata in domainspecific digital libraries of one domain (aggregations of cultural heritage digital collections
that are created for history scholars, educators, and enthusiasts) and of two scales – national
and international. The task of developing best practice guidelines warrants more extensive
content analysis of collection-level subject metadata, including those in domain-specific
digital libraries with a subject focus other than history (e.g., National Science Digital
Library) or non-domain-specific digital libraries with wide subject coverage (e.g., IMLS
Digital Collections and Content Collection Registry), with different scale (e.g., state-level
aggregations such as Missouri Digital Heritage or regional-level aggregations such as
Mountain West Digital Library, Documenting the American South etc.), and representing
geographic areas beyond Europe and North America (e.g., New Zealand Digital Library).
ACKNOWLEDGMENT
The author wishes to thank developers of The European Library, American Memory,
and Opening History digital libraries for providing collection-level metadata for this analysis.
Special thanks to Drs. Carole L. Palmer, Allen Renear, and Kathryn La Barre at the
University of Illinois (USA) and Dr. Dietmar Wolfram at the University of Wisconsin (USA)
for valuable feedback on this study.
BIOGRAPHICAL NOTE
Dr. Oksana L. Zavalina is an Assistant Professor at the Department of Library and
Information Sciences, College of Information, at the University of North Texas in the USA.
She was born, raised and received undergraduate library and information science (LIS)
training in Kiev, Ukraine. Dr. Zavalina’s two graduate degrees in LIS come from the
University of Illinois at Urbana-Champaign (UIUC): MLIS in 2002 and Ph.D. in 2010. Her
previous positions included being a bibliographer at the National Parliamentary Library of
Ukraine, a solo librarian at the Kiev School of Economics, an intern at Yale University
Sterling Memorial Library, an original cataloger at the UIUC Library (including with Google
Digitization Project), and a research assistant with the Center for Informatics Research in
Science and Scholarship in Illinois. Dr. Zavalina has been teaching graduate-level courses in
library cataloging and classification since 2007, in metadata and in information organization
since 2011. Her research focuses on subject metadata and its role in subject access in digital
libraries.
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