Psychosocial interventions, learning interventions, or interventions with permanent effects cannot be adequately assessed with crossover study designs due to irreversible carryover.

“If you're dealing with things other than that where you can't have a long enough wash out or it might be a psychosocial effect or a learning effect or some permanent effect, right? Anything from buriatric surgery to um something like an allergen which you know may permanently sensitize the body. Uh I know people mRNA vaccines hey maybe the mRNA hangs around for a while. is that a permanent effect or a vaccine itself is a you know we hope in some cases is a permanent effect like a measles vaccine. Uh all of those things they're not going to wash out.”