There are both practical and theoretical difficulties in determining h. The practical difficulties can be illustrated by the fact that <answer>the two most accurate methods, the watt balance and the X-ray crystal density method, do not appear to agree with one another</answer>. The most likely reason is that the measurement uncertainty for one (or both) of the methods has been estimated too low – it is (or they are) not as precise as is currently believed – but for the time being there is no indication which method is at fault.
What is a main practical difficulty in determining n?
the two most accurate methods, the watt balance and the X-ray crystal density method, do not appear to agree with one another