well the pinch test, which is on the side of your stomach, triceps and back only cover part of your body. as for the water displacement test, what about the air in your lungs? and the handheld tests just send a current through your body, which will never be 100% accurate, since sugar or anything small can easily mess it up by a few percent. and the ones you just put in your waist size or something is just an estimation for you, the same goes for the BMI test.