Biology Reference
In-Depth Information
k
groups
a priori
(
k
2 for sex estimation), the centroid of each group is determined and
each specimen is reassigned to the nearest centroid. Then an iterative process is initiated
in which a new group centroid is computed and specimens are re-assigned to the nearest
centroid. The process continues until group memberships and the estimates of the cen-
troids are stable. The resulting rate of assignments is assessed in the same manner as a
CVA or discriminant analysis.
The assignment rates for the Gonzalez et al
.
study are shown in
Tables 14.3 and 14.4
.
The overall cross-validation rates (frequencies of correct assignments) based on discrimi-
nant analysis of shape data only were 90.9% for the sciatic arch and 93.4% for the ischio-
pubic arch. The
k
-means rates were 90.9% and 90.1%. Females were misclassified slightly
more often than males using ischiopubic data in both methods of analysis; combining
ischiopubic and sciatic notch data yielded a slight improvement in classification rates for
the discriminant function but not for
k
-means clustering. In this last result, the misclassi-
fied females were the three youngest but the misclassified males spanned a wide range of
ages. Interestingly, including size in the analysis usually yielded worse cross-validation
rates (
Table 14.4
) indicating that size was not helpful in forming these classifications.
Classification rates based on visual scoring rubrics have been reported as ranging from
96% correct (
Phenice, 1969
)to60
5
90% correct (
Bruzek, 2002; MacLaughlin and Bruce, 1990
).
Methods based on traditional morphometrics measurements have produced rates from 73%
to 79.1% (
Patriquin et al., 2003; Steyn and Iscan, 2008
). Because these rates are probably
TABLE 14.3
Rates of Correct Assignment of Specimens to Gender Based on Sciatic Notch and Ischiopubic
Complex Shape Using Discriminant Analysis and k-Means Clustering
Discriminant Analysis
k
-Means Clustering
Correct
Assignments
Incorrect
Assignments
Percent
Correct
Correct
Assignments
Incorrect
Assignments
Percent
Correct
Sciatic Notch
F
47
5
90.4
46
6
88.5
M
63
6
91.4
64
5
92.3
Total
110
11
90.9
110
11
90.9
Ischiopubic Complex
F
46
6
88.5
45
7
86.5
M
67
2
97.1
67
2
97.1
Total
113
8
93.4
112
9
90.1
Sciatic Notch & Ischiopubic Complex
F
49
3
94.2
46
6
88.5
M
65
4
94.2
65
5
94.2
Total
114
7
94.2
111
11
91.7
Data taken from
Gonzalez et al. (2009)
, Table 2.
