*5.4. Selection Effects*

Svinkin et al. [69] suggested that T50, the time during which 50% of the counts above background are recorded, is a more robust duration measure than T90, since it may be less affected by detector energy ranges. To eliminate possible selection effects and to verify the two-component solutions, the clustering analysis was repeated using T<sup>50</sup> as the duration parameter. For *Swift*/BAT, the initial MCLUST fit returned a three-component solution similar to Figure 1b—the short duration group remained the same, while the two long duration groups also exhibited the clear-cut spherical feature identified in the T<sup>90</sup> analysis. When clustCombi was applied, a two group-solution was the best fit. Group 1 and Group 2 were identical to the groups found in the T<sup>90</sup> analysis. Thus, for *Swift*/BAT, this method did not favour T<sup>50</sup> over T<sup>90</sup> as a duration measure, and the results further supported the two-group solution.

For *Fermi*/GBM, the initial MCLUST fit identified an extra long duration group in a fourcomponent solution. The long duration group in Figure 5b was split in two, with the remaining structure matching the results of the T<sup>90</sup> analysis. clustCombi resulted in a twocomponent fit closely resembling the structure and makeup of Group 1 and Group 2 of the T<sup>90</sup> fit. However, Group 1 contained ≈ 100 more GRBs than the T<sup>90</sup> fit. For *Fermi*/GBM, the two-component fit was supported, and while the T<sup>50</sup> parameter returned slightly different proportions in each group, it did not demonstrate any clear advantage over T<sup>90</sup> as a duration parameter.
