Question about use of Poisson probabilities

[Dora Smith]

My boss wants me to use poisson probabilities to compute the liklihood of
meeting various goals in relation to our project, where the average number
of records per month is 617,000.

I am having trouble getting Excel to compute probabilities as other than 0
or 1, though when I do examples on the web that involve very small numbers I
get teh correct answers with no trouble.

Are poisson probabilities intended for this use? Or are they only
applicable for small numbers of discrete events, like the liklihood that 4
cars will run a traffic light in a day?

What would be the appropriate statistic for the probability of meeting a
goal of 700,000 discrete events in a month, or a few million in a year?

 

[Jerry W. Lewis]

Elaborate on what you mean by "discrete events". Are any contigent on
others? At what rate do these events typically occur? Is that rate constant
over time? Are each of the events equally likely? ...

Assuming that a Poisson model is correct, native Excel functions are not
sufficiently accurate to work with numbers this large. If you still want to
do the work in Excel, you should download and use Ian Smith's vba library of
probability functions
http://members.aol.com/iandjmsmith/Examples.xls

Jerry