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I was taking a closer look at this table in the Wikipedia article https://en.wikipedia.org/wiki/Normal_distribution#Standard_deviation_and_coverage.

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It is the 4th column labeled or 1 in p that I'm having trouble with. It seems to be the inverse of the 3rd column, 1-p. What is the meaning of or 1 in p?

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    $\begingroup$ This is the value of $1/(1-p)$, there is one chance out of $3.1514$, $21.978$, &tc. to fall outside the interval. $\endgroup$
    – Xi'an
    Nov 21 at 6:04
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It is indeed the inverse of $1-p$. The column is mislabelled, since $p$ is already defined.

They are writing $1-p$ as a fraction $1/M$, and telling you $M$. It can get hard to compare very small fractions intuitively, but writing them as $1$ in $M$ can be more intuitive (e.g. "how many times would we need to repeat this sampling from a normal, to expect to see one value at least this many SDs from the mean?". Framed this way, it's effectively a "return period", which is a common way to express rare events like floods or extreme storms.)

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