Minecraft wiki entry for Miscellaneous, associated text: With only two items in the dropper, both output power levels are chosen with equal probability. The probability of the output levels can be changed by adding additional stackable and non-stackable items to the dropper (which must all be different from each other so they don't stack). For example, with two different stackable items and three different non-stackable items, the RNG outputs power level 1 40% of the time and power level 3 60% of the time.