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What is the Epoch Capabilities Index (ECI)?
Last reviewed July 16, 2026
The Epoch Capabilities Index (ECI) is a single capability score fit across many models and 50+ benchmarks using item response theory: benchmarks differ in difficulty, models differ in ability, and the statistical fit estimates both at once. Published openly by Epoch AI (data under CC-BY), it makes models comparable on one scale even when no two ran identical test sets, and it carries confidence bounds, so uncertainty is visible instead of hidden.
How an IRT fit works
Item response theory, borrowed from standardized testing, models the probability that a model of a given ability passes a benchmark of a given difficulty. Feeding it many models' results on 50+ overlapping benchmarks yields difficulty estimates for the tests and ability estimates for the models simultaneously. New results refine the whole fit, and models that never ran the same benchmark still land on one scale.
What the index cannot tell you
A composite capability score hides shape: two models with equal ECI can differ widely on coding versus math versus factuality, and the index says nothing about price, speed, or context window. Confidence bounds matter too: sparse evaluations mean wide intervals. Use ECI to shortlist, then check the named benchmarks and live numbers closest to your work.
Top models on Epoch Capabilities Index
Full leaderboardPeak scores from the compiled benchmark data; reasoning models use their highest effort tier.
idapt's capability rankings and model pages use ECI rescaled to 0-100, re-fit for catalog models Epoch has not scored, always shown with confidence bounds and a provisional marker where the interval is wide, and never blended with price.
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