--depending on where they are being done, the skill of the technician/clinician, the placement of the electrodes, and even the temperature in the room (the test is supposed to be done under relatively cool conditions, as heat is the enemy of nerve conduction).

This, of course, doesn't even begin to take into account the possibility that the condition of the nerves themselves can change over time.

Just from the standpoint of consistency, I think it best to have these done by the same facility and the same people with the same equipment over time for purposed of comparison; I realize with insurance considerations and the like this may not be possible.