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Equation 28 · Part 1 · Why Average Success Rate Hides the Failures That Matter Most

Symbol q_τ

qτ=10−5q_\tau = 10^{-5}
qτq_\tau

What this part means

q_τ is part of the quantity the equation computes from the expression on the right.

Its job in the formula

q_τ is part of the quantity the equation computes from the expression on the right.

The passage around this formula

…is about the spread of a difference, not the rarity of an event. Bounding or estimating a rare catastrophic-failure rate is different in kind: the required sample size is inversely proportional to the rate itself. At qτq_\tau = 10^{-3} , getting even a rough twenty-event estimate needs on the order of twenty thousand trials. At qτq_\tau = 10^{-5} — the dangerous-failure probability ceiling regulated safety-critical software is held to under the SIL 4 standard, a bar Rabanser and colleagues cite alongside the FAA’s…

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A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.

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