## 4 Results
HV was used 18 times, IGD was used six times, and there were nine other additional metrics in this set, which were the hypercube-based diversity metric, which was used five times, and delta spread (\(\Delta\)) and ED, which were used four times each. The remaining metrics were used as follows: \(\epsilon\) and S were used three times each, GD two times, and finally, HVR, NDS, and \(\rho\) (convergence measure) were used only one time each. Figure 4: Metrics distribution in M1 group. Figure 3: Publication distribution for the study groups. Figure 6 shows a total of 16 metrics that appeared in the publications. After comparison according to their usage, HV was found to be the highest in total with seven cases, while NDS and the hypercube-based diversity metric were ranked five and four, respectively. While GD, the contribution metric, \(\Delta\), and CM were used two times each, and the remaining nine metrics in this list had the lowest values, only having been used once. Figure 5: Metrics distribution in M2 group. Figure 6: Metrics distribution in M3 group. In the M4 group, as Figure 7 shows, GD is the most used for the first time, having been used five times, and HV and IGD are in second position, having been used four times. In this graph, 11 unique metrics are involved with a total of six articles in the set (M4) and a two-set coverage (C). Spacing (S) metrics gained three and two, respectively, while the rest of the metrics had one use in each, which are HVR, PFS, GS, the hypercube-based diversity metric, \(\Delta\), and ED metrics. In Figure 8, although the articles utilizing more than two metrics are lower in number, the number of unique metrics is high. The figure indicates 15 unique metrics with the frequency used for each metric and their scores as follows: HV was used three times, and MS, \(\epsilon\), and S were used two times each, while the rest (HVR, GS, ER, IGD, GD, R2, \(\Delta\), CM, D, C, and IGD+) were reported only one time each. Figure 8: Metrics distribution in M5 group. Figure 7: Metrics distribution in M4 group. Finally, articles that employed six metrics (M6) were also fewer in quantity (four in total), and they used ten unique metrics. Figure 9 shows the frequencies of these metrics. HV and IGD had four each, and the remaining eight metrics were reported with three different scores: PFS, GS, and \(\epsilon\) had three each, ER and GD had two each, while the contribution metric, MPFE, and S were reported once each. Although the total of the unique reported metrics is 27 out of 105 articles, and they are repeatedly used in some of them. From this perspective, we can answer another research question on the total ranks of these 27 metrics over the studies. RQ3: What is the rank order of the metrics most or least used in the studies? To show the overall metrics with their ranks by calculating the total frequency of each metric, the above-mentioned analysis was used to determine how group studies employed these metrics in separate representations. Thus, in this section, the most or least used metrics (high or low in frequency) are described using their total frequencies. Table 3 shows the 27 unique metrics grouped based on their frequencies in column two. This means those that received the same value will be in the same set or rank. Column three shows the percentages calculated for each metric as the product of the frequency/total frequency of 168 multiplied by 100. Please note the total of the percentage values should be calculated as follows. For example, frequency number five has three metrics in that position (PFS, GS, C), and each of them has the value 3.0%, which means their total must be calculated as 3.0 + 3.0 + 3.0 = 9, and the rest should be calculated in the same manner (only if a set of metrics is in the cells) to reach a total of 100%.