If two miners mine the next block at similar times there will be a "fork" in the chain, with one block being accepted, and the other one becoming "orphaned".
What is the probability for this event to occur on the Bitcoin blockchain?
Looking at the orphaned blocks list I can see it is not something so rare, but I'm not able to quantify it. By intuition, I'd calculate the average number of blocks between the orphaned blocks, that is "every how many blocks there is an orhpaned block, on average". Given that, I can say that for each new block, there is a probability P for that block to become orphaned.
Is this approach correct? If so, how can it be stated formally?