Different measurement positions lead to different measurement results

Different measurement positions lead to different measurement results                                                                                                                                                                                                                                                                                                                   Many of us would have seen the picture of four oximeters on four fingers showing four different readings. It has some shock value, but it depicts a natural phenomenon: variation.                                                                                                                                                                                                                  It is known scientifically as variability (uncertainty) in measurement. It is distinct from ERROR; it refers to the intrinsic variation which is present in the system. All aspects contribute to it: the object (patient), the subject (doctor), as well as the instrument.                                                                                                                                                                                                              Often, this is reported in two ways: (1) keeping the object and the instrument same the readings are taken by different subjects, and (2) keeping the object and the subject same, the readings are taken on different instruments.                                                                                                                                                                                                                                Both the 'spreads' in readings indicate the 'built-in' variability of the measurement system. It can be large or small, but is never zero.                                                                                                                                                    We should therefore always remember that, apart from inaccuracy of an instrument, there is a contribution due to variability of measurement system which takes the result away from the true value. Incidentally, this subject of Measurement Systems Analysis is a part of requirements of 6 sigma, and forms a part of calibration exercise.                                                                                                                                                                                                                                                                                                                                                                                                                                                                      The user,therefore, should not take any reading (even his own) as the 'true value'. A mature, experienced doctor always 'factors in' the incoming data with patient's background, history, the lab where the test were conducted, the frequency of the tests, the frequency of calibration of instruments, the number of results in hand etc., etc., before arriving at final decisions about treatment of the patient.