Does Validation Loss Predict Real-Robot Success? Evidence from Four Imitation-Learning Policies on a Low-Cost Arm
arXiv:2611.00283v1 Announce Type: new Abstract: Held-out validation loss is the standard offline signal for choosing checkpoints and comparing models, but its relation to closed-loop success on a real robot is rarely measured. We measure it for ACT, Diffusion Policy, SmolVLA and $\pi_{0.5}$ (action expert only), trained on nested subsets of 12 to 200 demonstrations of a pick-and-place task on an SO-101 arm. We score 82 checkpoints on 25 held-out demonstrations, with each policy's training loss and with the error of the actions it outputs, and compare the scores with 930 bench rollouts. We also score all 700 intermediate checkpoints of 70 training runs, and recover the ball positions of every training, held-out and bench episode from the front camera. Within each policy, hold-out action error correlates with bench success (Spearman $\rho$ from -0.23 to -0.88), but no more strongly than the number of training episodes alone, and not at all among seeds trained on the same data. Across pol

