Figure shows the distribution of the EXT_CHEM and EXT_PROT for the 157 test tasks of the FS-Mol benchmark set using the default modeling setup. The key observation from the plots in Figure was that the hardness values have sufficient dynamic range (meaning the values are not concentrated within a narrow range) to be discriminative. The discriminative power is essential for using hardness as a predictor of model performance. Together, these results indicate that EXT_CHEM is robust to small-molecule representation. To quantify the sensitivity of the EXT_PROT to protein representations, we again calculated Spearman's r between the default protein representation, ESM2-t33-650M (a transformer pre-trained with the masked language modeling task), and all other explored descriptors (ESM2-t6-8M, ESM2-t12-35M, ESM2-t30-150M and ESM2-t36-3B; see Methods for details). As shown in Figure , all combinations yielded Spearman's r > 0.86, with one exception. The exception is the comparison of ESM2-t33-650M with ESM2-t6-8M, a small transformer model with six layers. The Spearman's r was just 0.78 for this combination, likely because the number of layers and parameters for this combination and, consequently, the generated representations were also very different. Overall, methods based on a similar number of layers and parameters showed higher correlations than those with substantially different numbers of layers and parameters. These results confirm the robustness of the EXT_PROT metric concerning protein representations. Intuitively, low values of k should increase the sensitivity of the task hardness measure to outliers. For example, if k=1 and there is just a single source task that is highly similar to the target task, then the task hardness would be very low (which is not the desirable behavior, because meta-learning is expected to have the largest benefits over single-task models in cases where several of the source tasks are related to the test task). High k values are also expected to diminish the hardness metric's discriminatory power.