Single-subject researchers address external validity concerns by relying on the strong, consistent effects they observe, which are likely to generalize even from small samples. Additionally, they place a strong emphasis on replication; by attempting to reproduce effects with different participants or under slightly varying conditions, they systematically build confidence in the generalizability of their results. This methodological approach is supported by the historical success of principles like classical and operant conditioning, which were primarily discovered using single-subject research and have been successfully generalized across an incredibly wide range of species and situations.