Finally, there is a more research-intensive approach that involves interviewing students for the purpose of generating the items that will make up a concept inventory or other forms of diagnostic instruments. Concept inventories require intensive validation efforts. Perhaps the most influential of these concept inventories to date has been the Force Concept Inventory (FCI). Concept inventories can be particularly helpful in identifying difficult ideas that serve as a barrier to effective instruction. Concept inventories in natural selection and basic biology have been developed.