Make and keep your product backlog DEEP. This acronym stands for detailed appropriately, estimated, emergent and prioritized and was introduced by Roman Pichler. To have it detailed appropriately means to have the top items well refined and with more detail than the lower items. To have it estimated is key to maximize the value delivered. To be emergent means that the backlog is dynamic and constantly refined. To have it prioritized means to ensure that the highest value items will be delivered first.