# Open models lag closed models by 4 months > Source: > Published: 2026-05-31 04:02:46+00:00 Epoch's work is free to use, distribute, and reproduce provided the source and authors are credited under the Creative Commons BY license. ## Learn more about this graph We calculate the average amount of time it takes for the best open-weight models to catch up with state-of-the-art performance according to our internal capability metric, the [Epoch Capability Index](https://epoch.ai/eci) (ECI). ECI is a composite measure that captures performance across many benchmarks. ### Analysis ### Limitations ## Explore this data Benchmark results featuring the performance of leading AI models on challenging tasks.