Minecraft wiki entry for Miscellaneous, associated text: With only three items in the dropper, all three output power levels are chosen with equal probability. The probability of the output levels can be changed by adding additional 64-stackable, 16-stackable, and non-stackable items to the dropper (which must all be different from each other so they don't stack). For example, with one 64-stackable item, one 16-stackable item, and two different non-stackable items, the RNG outputs power level 1 25% of the time, power level 2 25% of the time, and power level 4 50% of the time.