sensors are not always super independent so you might have a sensor and you might say okay maybe I have a laser rangefinder and it reads a reading but then maybe send out two beams next to each other but those two beings might encounter the same kind of noise or the same kind of interference in their paths and so those two measurements might not be independent and the assumption we just went through where there's all simplifies just becomes a product of things is violated even though often in the math will want this and practice be