Read this: There are both practical and theoretical difficulties in determining h. The practical difficulties can be illustrated by the fact that the two most accurate methods, the watt balance and the X-ray crystal density method, do not appear to agree with one another. 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.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What difficulties are not involved with determining h?
unanswerable