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).