Title: Confidence and Dispersity Speak: Characterising Prediction Matrix for Unsupervised Accuracy Estimation. Abstract: This work focuses on estimating how well a model performs on out-of-distribution (OOD) datasets without using labels. Our intuition is that a well-performing model should give predictions with high confidence and high dispersity. While recent methods study the prediction confidence, this work newly finds dispersity is another informative cue. Confidence reflects whether the individual prediction is certain; dispersity indicates how the overall predictions are distributed across all categories. To achieve a more accurate estimation, we propose to jointly consider these two properties by using the nuclear norm of the prediction matrix. In our experiments, we extensively validate the effectiveness of nuclear norm for various models (e.g., ViT and ConvNeXt), different datasets (e.g., ImageNet and CUB-200), and diverse types of distribution shifts (e.g., style shift and reproduction shift). We show that the nuclear norm is more accurate and robust in predicting OOD accuracy than existing methods. Lastly, we study the limitation of the nuclear norm and discuss potential directions.