Left: row-normalized rotation-velocity–[Fe/H] diagrams comparing the classical selection (red) with the refined criteria (green). The first panel shows the population boundaries from V. Belokurov et al. (2020), and the second panel presents the distribution of all high-α subsample. The remaining panels show the distributions in different stellar age bins. In the oldest bin (bottom right), the refined criteria adopt a lower metallicity threshold than the classical method, and the upper velocity limit of the Splash selection falls below 100 km s⁻¹. Toward younger ages, the refined Splash selection shifts to higher metallicity and higher rotational velocity. Right: number ratios of Splash to thick-disk stars as a function of stellar age. With the classical selection (red), the ratio declines steadily beyond 9 Gyr. In contrast, the refined method (green) yields an approximately constant ratio at the oldest ages, consistent with our assumption that Splash stars were produced with similar probability across stellar ages.