David Allison

Or it could be an effect of what treatment A and treatment B did in the first period carrying over.

And now you don't have a clean estimate of the effect of treatment A or treatment B anymore.

Now what you can do is what you said is a washout.

If you said, I understand how this thing works.

And it's a molecular effect.

It's not a social effect.

It's not learning.

It's not a surgical thing.

I didn't cut a piece of their anatomy out that doesn't reverse.

This thing is completely reversible.

It's completely blinded.

I gave a long enough washout.

Then by that argument, you can say I rule it out.

But you can never say absolutely rule it out.

If you're dealing with things other than that, where you can't have a long enough washout, or it might be a psychosocial effect or a learning effect or some permanent effect, anything from bariatric surgery to something like an allergen, which may permanently sensitize the body.

I know people say mRNA vaccines.

Maybe the mRNA hangs around for a while.

Is that a permanent effect?

Or a vaccine itself is a, we hope in some cases, is a permanent effect, like a measles vaccine.

All of those things, they're not going to wash out.