The table shows the cumulative odds of getting at least one item given a 0.1% drop rate (one in a thousand)

I.E. a 1/1000 drop rate does not mean that if you kill 1000 you WILL get one; rather since the chance is 0.1% each time you kill the mob, then the odds of getting at one from N kills is 100% minus 99.9%N. Furthermore, since the cumulative odds never quite reach 100%, it is possible—albeit unlikely—to never get one.

Probability of at least one drop in X kills
Number of Kills 100 288 401 693 1386 2302 2995 4603 9206
Cumulative Probability 9.5% 25% 33% 50% 75% 90% 95% 99% 99.99%
Neither the drop rate nor cumulative probability increases based on the number of monsters you have already killed.
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