Because the magnitude of many of the units comprising the SI system of measurement is ultimately defined by the mass of a 137-year-old, golf-ball-sized piece of metal, the quality of the IPK must be diligently protected to preserve the integrity of the SI system. Yet, despite the best stewardship, the average mass of the worldwide ensemble of prototypes and the mass of the IPK have likely diverged another 6.4 µg since the third periodic verification 27 years ago.[Note 18] Further, the world's national metrology laboratories must wait for the fourth periodic verification to confirm whether the historical trends persisted.