Application of
Forensic Discriminant Functions to a Spanish Cranial Sample
Douglas
H. Ubelaker
Curator
Department of Anthropology
National Museum of Natural History
Smithsonian Institution
Washington, DC
Ann H. Ross
Postdoctoral Associate
C. A. Pound Human Identification Laboratory
University of Florida
Gainesville, Florida
Sally M.
Graver
Research Assistant
Department of Anthropology
National Museum of Natural History
Smithsonian Institution
Washington, DC
Introduction.......Materials
and Methods.......Results.......Classification
of Race
Discussion.......Conclusion.......References
Introduction
Since the 1960s,
forensic scientists have been aware of the importance of discriminant
function analysis of measurements to assist in the estimation of
sex and ancestry of human remains (Giles 1964; Giles and Elliot
1962; Hanihara et al. 1964; Steel 1962). Such functions offer powerful
classification approaches once investigators have learned to record
measurements and use the functions properly.
The forensic
applications of discriminant function analysis were augmented considerably
in recent years with the development of FORDISC 2.0 (Ousley and
Jantz 1996). This interactive computer program offers custom discriminant
functions for up to 21 cranial measurements. Unlike most previously
published functions, this program allows classification, even with
incomplete remains for which only a limited number of measurements
is possible.
The reference
samples used in FORDISC 2.0 are based on data recorded in the Forensic
Data Bank (Jantz and Moore-Jansen 1988; Moore-Jansen et al. 1994).
These data were collected from identified forensic cases, making
them ideally suited for forensic applications. FORDISC 2.0 uses
over 1,400 cases from the Forensic Data Bank, offering more relevant
information than many of the previously published discriminant functions
based largely on museum collections. FORDISC 2.0 also classifies
individuals based on the Howells database (Howells 1973; Howells
1989) of cranial measurements taken on museum collections of archeologically
recovered remains from around the world.
The discriminant
functions derived from these two databases offer different, somewhat
contrasting approaches to ancestry classification. The Forensic
Data Bank includes categories of American black males and females,
American Indian males and females, American white males and females,
Chinese males, Japanese males and females, Vietnamese males, and
Hispanic males (from the United States, Mexico, and Central America,
but mostly representing Mexican Americans). Unknown remains are
classified into the racial categories that are represented within
the Forensic Data Bank. The race categories are then based on known
information about the identified individuals within the database.
Ousley and Jantz (1996: 20-24) provide an excellent discussion of
these social race categories.
The Howells
groups are those used in his 1973 and 1989 publications and largely
reflect the names assigned to the archeological samples examined.
Since these are older samples and more geographically diverse than
those in the forensic database, they offer a different perspective.
Discriminant
function classification assumes that the unknown originates from
one of the reference samples within the database. Thus, the utility
of the software in a forensic context depends largely on the similarity
of the samples in the database with the population of the unknown
(Birkby 1966). Although it would be surprising if an unknown forensic
case originated from any of the specific Howells samples, the option
could offer some potentially useful insight into the likely ancestral
origins of the unknown individual.
FORDISC 2.0
has emerged as a powerful tool in the forensic analysis that is
routinely employed in most North American forensic laboratories.
Because the Forensic Data Bank was developed largely from North
American forensic cases, its obvious use would be in applications
to North American cases (Ubelaker 1998). However, Ousley and Jantz
(1996:19) caution against applying it to individuals whose "race
or ethnic group is not represented in the reference sample."
The caution becomes more apropos as the international use of the
program increases. With the increase in today's world travel, a
representative of almost any population in the world could end up
being a forensic case in almost any place in the world.
Materials
and Methods
The Departamento
de Biología Animal of the Universidad Complutense of Madrid,
Spain, curates a skeletal collection from Wamba, near the Spanish
towns of Villanubla and Valladolid in northwestern Spain. The collection,
likely dating from the 16th to 17th century, originates from a large
secondary ossuary deposit in the Church of Santa Maria and represents
individuals affiliated with that church (Lopez de los Bueis 1998;
Perez de Barradas 1952). Through the courtesy of curator Maria Dolores
Garralda of the Universidad Complutense, this sample was made available
to the authors for measurement during the summer of 2001. The 20
measurements listed in Table 1,
as defined by Moore-Jansen et al. (1994), were used in the Forensic
Data Bank and this study. This large Spanish sample offers an excellent
opportunity to examine the application of FORDISC 2.0 to a diverse
sample not represented in the database.
Even though
this large sample represents a specific group at a particular time,
the authors, during their examination of the remains, were impressed
with the morphological heterogeneity within the group. Measurements
were recorded on 95 individuals in this sample. Of these, 58 were
assessed to be male and 37 female using gross morphological indicators
(Bass 1995; Ubelaker 1999). The measurements for each cranium were
entered into FORDISC 2.0 and classified using both the Forensic
Data Bank and Howells samples. Each cranium was then entered into
FORDISC 2.0 and analyzed twice, once using reference groups of both
sexes and once using reference groups of the assigned sex.
Results
As shown in
Table 2, all of the 37 crania that
the authors estimated to represent females were also classified
as females by both the Howells and Forensic Data Bank options. In
contrast, of the 58 crania the authors estimated to represent males
using morphological indicators, 33 (57 percent) were classified
as females using the Forensic Data Bank, and 30 (52 percent) were
classified as females using the Howells database. Because the Spanish
crania tend to be comparatively small, the sex discrepancy likely
represents FORDISC's classification solely by measurement without
considering non-metric observations.
Classification
of Race
All of the crania
in this Spanish sample originated from a 16th to 17th-century community
associated with a church in northwestern Spain. Although specific
information on the ancestry of any particular individual in the
sample is not available, generally all individuals should be considered
examples of this group.
Table 3 presents the race classification
of all individuals in this sample using the Forensic Data Bank option.
Of the 95 individuals, 42 (44 percent) were classified as white,
35 percent as black, 9 percent as Hispanic, 4 percent as Japanese,
4 percent as American Indian, and the remaining three individuals
as Chinese and Vietnamese. Using the sex estimates made by the authors
at the time of examination as Sex Known and classifying the crania
by race within the specific sex category using the Forensic Data
Bank option, the greatest percentage classified this time into the
black category. However, the distribution of the other groups was
similar to the race, Sex Unknown classification. The comparison
of Sex Known racial classifications with Sex Unknown racial classifications
reveals that group classification changed for 17 estimated males
and 3 females. Note in Table 3
that the Forensic Data Bank samples of Hispanic, Vietnamese, and
Chinese consist only of males.
Table
4 illustrates that, using the Howells database, the 95 individuals
were classified into 21 different groups. The most common group
was Egyptian, followed by a medieval sample from Austria. The 21
different groups represent samples from 15 different countries of
Australia, Asia, Africa, Europe, and the Americas. Note that Table
4 includes only the classified groups of the Spanish sample.
Using the Sex
Unknown option, group classifications using the Howells database
were different from those using Sex Known (estimates of sex made
by the authors) in 40 percent of males and 11 percent of females.
The Egyptian group was the most frequently classified in both Sex
Unknown and Sex Known attempts. Fifty-eight percent of crania from
the Spanish sample were classified into European or North African
groups using the Sex Unknown search and 55 percent using the Sex
Known information.
Table
4 also presents the typicality probabilities for the group classifications
of the Spanish sample. This statistic offers information about the
likelihood that a particular skull actually belongs to the group
under discussion. Generally values less than .05 are considered
atypical and those greater than .05 typical.
Crania classified
into European or North African groups were considered typical in
63 percent of classifications using Sex Known and 69 percent using
Sex Unknown. Typicality values were slightly greater for crania
classified into other groups (non-European or North African) with
values of 74 percent for Sex Known and 73 percent for Sex Unknown.
Of the 65 crania
of Sex Known with typicality values greater than .05, 51 percent
were classified as European or North African and the remaining 49
percent as others. Of the 68 crania of Sex Unknown with typicality
values greater than .05, 56 percent were classified as European
or North African and 44 percent into other groups.
Discussion
The attempt
to classify the Spanish sample in terms of sex and race produced
a variety of results. The sample displayed considerable heterogeneity
and was not directly represented in either of the databases used
in FORDISC 2.0. Differences between sex estimates made morphologically
by the authors and those generated by FORDISC 2.0 can be explained
largely by size factors within this Spanish sample. The results
demonstrate why all morphological indicators should also be consulted
in forensic analysis.
Variation in
racial classification represents the lack of a Spanish sample within
the FORDISC 2.0 databases as well as the human variation inherent
within them. Individual crania were classified according to the
best fit with the existing samples of the database, but the samples
clearly were inadequate to elucidate the specific geographical origin
of the overall Spanish sample. Although the actual ancestries of
the individuals within the Spanish sample are unknown, it is possible,
especially considering migration patterns across that part of Europe,
that some ancestral representatives of the groups suggested by FORDISC
0.2 may be represented. However, it is more likely that the Spanish
sample simply is quite different from samples in the existing databases.
A majority of the crania were classified into European or North
African groups. However, some crania were classified into groups
with no clear geographic or ancestral relationship with the Spanish
sample.
Conclusion
The authors
agree with Ousley and Jantz (1996) that caution is called for when
applying FORDISC 2.0, as well as other similar discriminant function
approaches, to samples that are not well represented in the relevant
databases. Investigators should remember that such approaches should
complement, not displace, overall evaluation of remains. The authors
also agree that additional and more complete samples from different
geographical regions and groups are needed to augment the existing
databases. Such additions would improve an already useful forensic
tool and make it more applicable to international forensic cases.
References
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