FIGURE 4 Illustration of the age group threshold search
protocol for different age threshold tests. The test 1 was used to find
a single threshold for dividing data into two classes, while the tests 2
and 3 utilized two thresholds for either excluding the data or creating
a third rejection class.
3 | RESULTS AND DISCUSSION
3.1 | Classification with pre-defined age groups
Table 2 illustrates the results of the classification to pre-defined age
groups. The classification between all four groups shows moderate
results with all classifiers reaching bacc between 66 – 67 %.
Interestingly, the highest performance is reached in the classification
task of groups A vs. D, where the age difference between the groups is
the largest. The best performing model confusion matrices for the
previously mentioned tasks are shown in the Figure 6. Furthermore, the
performance in binary classification task of younger groups A, B, C vs.
the oldest group D can be seen to decline linearly as the age gap
between groups decreases as shown in the Figure 5. The result indicates
that distinguishing the younger groups from the oldest group is better
when the age gap between the groups is higher. On the other hand, the
binary classification results between the groups A, B and C show poor
performance, suggesting similarity between the groups. The majority of B
and C classifications is done to the group A, which is likely affected
by the class imbalance, group A being the largest. With more balanced
class sizes, more equal proportion of miss-classified samples between
groups A, B and C would be expected in the case of suspected homogenous
samples. Overall, the results suggest no great difference between the
younger groups A, B and C in this study setup, while the age-related
differences become more distinct with the oldest age group.
TABLE 2 . The classifiers performance for the age group
classification test with pre-defined age groups. The results include
balanced accuracy and 95 % confidence intervals inside the brackets.
Polynomial SVM in A vs. D age group classification task achieved the
highest performance (highlighted in bold).