In healthcare research, parallel group Randomized Controlled Trials (RCTs) are currently considered the gold standard of research; and the more participants you have, the better. Why would a researcher want to waste time on a study that only has a sample size (N) of 1?
The answer is that, although these studies are designed so the participant feels like they are the only research participant, many of these trials can be run together to get detailed information on individually designed treatments. This information may suggest ways to better treat larger groups of individuals with similar characteristics.
N-of-1 trials are also useful for tracking interventions for rare diseases when it’s difficult to recruit enough people to run a standard RCT. In an N-of-1, the treatment can be tailored to just one person as well as document how an intervention affected this rare disease.