Ideally, the random input generated will match reality. If the average number of accidents per year is very unlikely to be less than 15 in real life, then the number generated in the simulation should be very unlikely to be less than 15. Sometimes you know that the distribution may be normal with known mean and SD. If you don't know the exact distribution, then a common compromise is the triangular. You set the maximum, minimum and most likely values. Then the simulation picks a random input from that range, most likely around the nominated values. So you might, after due consideration and consultation with experts decide that the average MVA/year is probably about 25, but because of our lack of hard data could possibly be as low as 20, say. or as high as 30. We could then use a triangular 20, 25, 30 and plausible scenarios would have MVA/y mainly round 25 but going as low as 20 and as high as 30. You are in effect modelling your ignorance of the real value.
You are great thanks for all your help!
I will go away and play with that tool with some phony data to see if I can get the hang of things!