[BruceZ]

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Very nice. Thank you for the info.

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For a normal distribution, the probability of an outcome at least 2.68 standard deviations below average is about 0.368%, or 1/272.

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Is this the sort of conversion I need to look up in a table? Are there any tools in, say, MS Excel that can do this part?

-Grendel

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Excel function =NORMSDIST(-2.68) returns 0.368%
Excel function =NORMSINV(0.368%) returns -2.68

Note that NORMSDIST gives you the area from minus infinity to -2.68, so it always gives you the probability of being between minus infinity and the entered number of standard deviations from the mean. If you put in +2.68 for 2.68 above average, it would give you the area from minus infinity to +2.68 or 1-0.368% = 99.632%, indicating a probability of 99.632% of being between minus infinity and 2.68 standard deviations above the mean. NORMSINV is the inverse of NORMSDIST.