Mark,

My example was per drawing [per day]. Let's just say your odds are 1 in 1000 of being selected on any 1 day. Over a 20 day period, your odds become additive --

1/1000 + 1/1000 .... or 20/1000 or 1 in 50.


Adding probabilities is not right: in 2000 days you would get 2 in 1, which is obviously ridiculous. You could either calculate the probability of not being selected: (999/1000)^20 is the probablity of not being selected in 20-day period. So, the being selected at least once: 1-(999/1000)^20 (about 0,20).



Auke