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Equation 7 · Part 10 · Measuring AI Agent Reliability: What the Evidence Actually Supports

Denominator: binomnk

pass@k  :=  Etasks ⁣[ 1  −  (n−ck)(nk) ]\text{pass@}k \;:=\; \mathbb{E}_{\text{tasks}}\!\left[\, 1 \;-\; \frac{\binom{n-c}{k}}{\binom{n}{k}} \,\right]
(nk)\binom{n}{k}

What this part means

The complete quantity below the fraction bar; it must be nonzero for this division.

Its job in the formula

binomnk occurs below the fraction bar. The quantity above the bar is divided by this expression; zero is excluded as a denominator.

The passage around this formula

Start with the metric that made repeated sampling a standard practice. When OpenAI’s Codex team evaluated a code-generating model against the HumanEval benchmark, they needed a way to score the strategy of drawing several candidate solutions from the model and keeping the best one. The obvious approach — generate exactly k samples per problem and check whether any of them pass — has an undesirable property: it is a valid estimate but a high-variance one, since it throws away information every time you happen to generate more or fewer than k samples. Their fix was to over-sample: draw n total samples per task, observe how many of them, c , actually pass, and then compute the exact probability…

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Learn the underlying idea

A fraction a/b means a divided by b. The top number is the numerator; the bottom number is the denominator, and it cannot be zero.

Open the illustrated fractions: division written vertically guide →

Sources cited in the surrounding passage

These citations provide research context; check each source for the exact claim it supports.