Maybe because you cannot guarantee that noise will bleed? If you detect bleeding, then great, you have a result. If you do not detect bleeding, then it could

Maybe because you cannot guarantee that noise will bleed? If you detect bleeding, then great, you have a result. If you do not detect bleeding, then it could be that there is a confounding variable or it could just be that the noise doesn't bleed. Similarly if there is a confounding variable then you cannot guarantee that the noise will bleed to both sets of observations. You have know way of knowing. Make sense?