On Nov 12, 9:44 am, Brakeshoe
wrote:
According to Excel's built in documentation RAND returns values greater than
or equal to zero and less than one. If this was true I believe I should
eventually see a random number equal to zero be returned.

Googling for "Excel RAND algorithm" (without quotes) found the
following explanation: http://support.microsoft.com/kb/828795 .

Arguably, the MS KB description is not dispositive because there is a
material error -- at least I presume so. It states that the algorithm
returns a random number "on [0,1]" (sic). Of course, the required
result is on [0,1), not [0,1].

On the other hand, if you look at the Fortran(!) implementation, we
see that the result is mod 1.0. That should not return 1.0. So
presumably the KB notation is simply a typo -- or written by someone
who is unfamiliar with the correct mathematical syntax.

Note that the KB article states that the RAND algorithm is capable of
returning "more than 10^13" pseudo-random numbers before repeating.
That is far fewer than the number of double floating-point values in
the range [0,1). I have not analyzed the algorithm to see if it can
return any number (or at least a large number) of sets of "more than
10^13" PRNs. Assuming that it can, as to whether or not zero is
included in any one set of 10^13 PRNs, that might depend, at least, on
how the PRNG is seeded.

This KB article does not explain that. I think there is another KB
article that does; but I am not taking the time now to track it down,
if indeed it exists. Typically, software-based PRNGs are seeded by
some manipulation of the time of day measured to the resolution
supported by the operating system.

HTH.