The largest studies are composed of multiple sites of participants. This has multiple advantages; for example, it makes it easier to recruit a diverse populati

The largest studies are composed of multiple sites of participants. This has multiple advantages; for example, it makes it easier to recruit a diverse population, reducing trial failures due to population-specific effects. Also, it helps randomize some aspects of treatment, so you don't depend too much on one site's special magic. If you can't recruit a large enough N of patients in one site, you might be able to recruit your N across multiple sites. There are downsides- overhead (each site has its own IRB that must approve the study) and additional complexity (the statistical modelling needs to be carefully managed). It's not uncommon in Cancer trials where there are multiple regional centers that specialize in cancer treatments, who see all the patients with specific types of cancer in a geographical region, and the prevalence of the cancer is high in the population. Also, if you want to change the usage for a drug, IE you were approved for lung cancer but now you want to use it for liver cancer- you might need to run another study. Not multi-site, but just a completely new study with the same medication, but new patients, new protocol (dosage and timing), new site location.