Minecraft wiki entry for Door-based_iron_golem_farming, associated text: Odds of a missed spawn occurring: There are 512 valid spawn points in the 16×6×16 block spawn area. Making a farm with only ½ of the valid spawn points (256) does not reduce your chance of spawn by ½. This is because the game randomly "tries" to find a valid place to put a golem 10 times each time an opportunity to spawn a golem comes up. So, your chances become 0.5 for each of the ten tries. Missing a spawn in this case is equivalent to flipping a coin ten times and have all ten come out tails. It is .5^10 or .00098 which is just about one in a thousand odds. A better suggestion would be to use some of the area of the zone to rapidly move golems out of the interference zone so they do not halt further spawning. This is a much more likely occurrence than missed spawns if the golems can't move out of the area quickly. This is not as rare of an occurrence as has been suggested.