Title: Beyond Model Ranking: Predictability-Aligned Evaluation for Time Series Forecasting
Primary area: learning on time series and dynamical systems
Keywords: Time-Series Forecasting; Spectral Coherence; Data Predictability; Model Evaluation

Abstract: In the era of increasingly complex AI models for time series forecasting, progress is often measured by marginal improvements on benchmark leaderboards. However, this approach suffers from a fundamental flaw: standard evaluation metrics conflate a model's performance with the data's intrinsic unpredictability. To address this pressing challenge, we introduce a novel, predictability-aligned diagnostic framework grounded in spectral coherence.
Our framework makes two primary contributions:  the **Spectral Coherence Predictability (SCP)**, a computationally efficient ($O(N\log N)$)  and task-aligned score that quantifies the inherent difficulty of a given forecasting instance, and the **Linear Utilization Ratio (LUR)**, a frequency-resolved diagnostic tool that precisely measures how effectively a model exploits the linearly predictable information within the data. We validate our framework's effectiveness and leverage it to reveal two core insights. First, we provide the first systematic evidence of "predictability drift'', demonstrating that a task's forecasting difficulty varies sharply over time.  Second, our evaluation reveals a key architectural trade-off: complex models are superior for low-predictability data, whereas linear models are highly effective on more predictable tasks. We advocate for a paradigm shift, moving beyond simplistic aggregate scores toward a more insightful, predictability-aware evaluation that fosters fairer model comparisons and a deeper understanding of model behavior. Codes and data are available at https://anonymous.4open.science/r/TS_Predictability-C8B7.

TL;DR: We introduce the first spectral coherence framework to disentangle model error from intrinsic data difficulty, enabling fairer and more diagnostic time series evaluation.