A comparison between measurements—one of known magnitude or correctness made or set with one device and another measurement made as similar as possible with a second device. The device with the known or assigned correctness is called the standard. The second device is the unit under test, test instrument, or any of several other names for the device being calibrated. Successful calibration has to be consistent and systematic. At the same time, the complexity of some instruments requires that only key functions be identified and calibrated. Under those conditions, a degree of randomness is needed to find unexpected deficiencies.