Minecraft wiki entry for Randomizers, associated text: With only three items in the dropper, all three output power levels will be 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 won't stack). For example, with one 64-stackable item, one 16-stackable item, and two different non-stackable items, the RNG will output power level 1 25% of the time, power level 2 25% of the time, and power level 4 50% of the time.