[1/*] If we take the chance that a tool (compiler, linker, batch, whatever) remains working for a particular codebase after one year as a given probability p, then the chance that build remains working after x years is p^xn, where n is the number of tools used in the build.
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Replying to @cmuratori
How did you come up with the p^xn formula and confirm its correctness? Is this specifically applied to the Twitter’s codebase or for any codebase for example aerospace or mission critical softwares?
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Replying to @tranmq
I did not "come up with" the formula - it is the only formula that can be applied given the assumption stated in the first clause of the tweet ("If we take... as a given probability p").
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Replying to @cmuratori @tranmq
If you have two probabilities (p and q), and you want to know if both will happen, you multiply them together. So "p*q" is the chance that _both_ p and q will happen.
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Replying to @cmuratori @tranmq
Ergo, if you have n tools, each with a probability of working p, then it is p*p*p*...*p, with n p's. Which is just p^n.
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Replying to @cmuratori @tranmq
Then you just repeat for the number of years, x. That's all the formula is.
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(In more sophisticated analyses, you would start looking at things like the dependency of p on q or vice versa, etc. This formula takes the assumption as a given, so it does not consider things like "p happening makes q more likely" or anything like that.)
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