Spillovers occur when one individual’s action of taking up a treatment impacts another individual, regardless of that individual’s assignment status. An illustrative example of spillovers are vaccines: If you are randomly assigned to be offered a vaccine—and you choose to take it up—you reduce the risk of others around you contracting the disease. It does not matter if the people around you are vaccinated or not—or even if they are in the study—the fact that you took up the treatment has impacted them.