Siginificant figures calculating zprob

[Paul Rubin]

sarah_wang23 at hotmail.com (Sarah Wang) writes:
> But the problem is that the function calculates the result
> with only a few significant figures. If I want to get the 
> 20th number of the result(z-prob) what should I do? I want
> to get the confidence level of "6sigma" and all I get at the
> moment is "1".

6 sigma is a marketing term.  The number you want is on the order of
10**-9, so it describes something almost never seen in real life.

http://en.wikipedia.org/wiki/Six_Sigma

> I remember that Python's long type has unlimited number of
> significant figures as long as the memory allows. What about
> floating point numbers?

That's for integers only.  Some math libraries have a function called
"erfc" (complementary error function) which does what you want.  You
can evaluate it to arbitrary precision in a system like Mathematica.
I think the number you want is 0.5*erfc(6/sqrt(2)) which according to the
Emacs calculator "calc" is 9.86587644935e-10.