Probability of 2 or less occurances is =POISSON(2,mean,TRUE)
Given that, there are various ways to answer your question:
- You can plot this by various values of mean and get a rough visual
estimate of what mean gives a value of 0.4.
- You can use Solver to numerically find the mean that makes this equal to
0.4
- You can download Ian Smith's probabilty library
http://members.aol.com/iandjmsmith/examples.xls
and use it to calculate =ucb_poisson(2,0.4)
- A less accurate implementation of the theoretically exact calculation for
a poisson upper confidence bound would be to use the native Excel functions
=GAMMAINV(1-0.4,x+1,1) or =CHIINV(0.4,2*(2+1))/2, where x=2 for your question.

The Poisson mean that has exactly 40% probability of 2 or less is 3.105379...

Jerry

" wrote:

hi all,
i would like to know whether i can / how to calculate the mean value
of occurrences from a given percentage probability for a given number
of occurrences.

for example, if know that there is a 40% chance there will be 2 or
less occurrences, how do i calculate the mean number of occurrences?

(i will then use this value as the mean part of various poisson
equations to work out the probability of various numbers of
occurrences) .

thanks for your help.