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Efficient Prediction of Pass@k Scaling in Large Language Models
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Proposes a beta-binomial fit of per-problem difficulty plus a dynamic (hardest-first) sampling allocation to predict pass@k at large k from small sampling budgets, outperforming log-log regression and discretized-beta fitting.
Problem
Predicting pass@k for very large k (repeated-sampling success or jailbreak rates) from a small sampling budget, where existing power-law regression and discretized-beta fits are statistically flawed and biased.
Approach
Model per-problem success probabilities with a beta distribution fit directly by beta-binomial maximum likelihood on variable per-problem counts (instead of discretized bins or log-log regression), and allocate samples adaptively to the hardest problems (fewest successes, then fewest attempts).
Key results
- Proposed beta-binomial plus dynamic sampling method achieves lower MSE than log-log regression and discretized beta across AdvBench, MATH, and Code Contests for budgets 10^1 to 10^4 (paper claims closest predictions to ground truth virtually everywhere).
- Log-log regression often predicts impossible pass rates above 1 at high k; discretized beta consistently underestimates pass@k at large k.
- Paper reports that for some models its MSE is more than 10x lower than counterparts.
Surprising findings
- Using the estimated scale parameter theta does not improve predictions empirically, despite the theory suggesting it could help.
Limitations
- Dynamic sampling benefits are hard to isolate empirically on the real datasets; main support comes from synthetic experiments.
- Theoretical variance-optimal allocation is derived for a frequentist estimator, not the Bayesian multi-stage estimator actually used.
- Ground truth relies on a withheld 10,000-sample set per problem, and data comes from prior work's models rather than newly sampled models.
Experimental setup
Datasets: AdvBench MATH CodeContests
Benchmarks: AdvBench MATH CodeContests
Metrics: mean squared error squared error of pass@k prediction
Baselines: Log-Log Regression Discretized Beta
Builds on: pass@k unbiased estimator discretized beta fitting (Hughes et al.) log-log linear regression power law fitting beta-binomial model
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