Lvthn posted...7/128 assuming you kill one of each, and not adjusting for minutae like additional enemies.

In fact that means that about 1 in 20 playthroughs should get a rare item on the first try.

This is not how probability works. While that makes sense intuitively, it's not mathematically correct. To find the probability you will get at least one 1/128 item, you actually need to find the probability that you will get -no- 1/128 items, and then subtract that from 100%.

Mathematically that works out to 1 - (127/128)^7, which yields:

1 - (0.9928775)^7 = 1 - .9466 = ~5.34%

7/128 yields 5.47%; I realize that's not a huge difference given the low sample set, but consider how the odds change if you take 64 attempts. 64/128 tells you that's a 50% chance you'll have at least one 1/128 item, whereas the proper expansion tells you the odds of finding one are actually ~37.5%. Going even higher, 96 attempts actually gives you ~52.9% odds to find one, whereas simply taking 96/128 gives you 75%.

Now you know. And knowing is half the battle!

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Setzerdog: What do you use guns for? The 13th Lyricist: Sometimes me and my son want to play catch and don't have a Frisbee, so the guns come in handy.