<P> Measurement errors can be divided into two components: random error and systematic error . </P> <P> Random error is always present in a measurement . It is caused by inherently unpredictable fluctuations in the readings of a measurement apparatus or in the experimenter's interpretation of the instrumental reading . Random errors show up as different results for ostensibly the same repeated measurement . They can be estimated by comparing multiple measurements, and reduced by averaging multiple measurements . </P> <P> Systematic error, however, is predictable and typically constant or proportional to the true value . If the cause of the systematic error can be identified, then it usually can be eliminated . Systematic errors are caused by imperfect calibration of measurement instruments or imperfect methods of observation, or interference of the environment with the measurement process, and always affect the results of an experiment in a predictable direction . Incorrect zeroing of an instrument leading to a zero error is an example of systematic error in instrumentation . </P> <P> The Performance Test Standard PTC 19.1 - 2005 "Test Uncertainty", published by the American Society of Mechanical Engineers (ASME), discusses systematic and random errors in considerable detail . In fact, it conceptualizes its basic uncertainty categories in these terms . Random error can be caused by unpredictable fluctuations in the readings of a measurement apparatus, or in the experimenter's interpretation of the instrumental reading; these fluctuations may be in part due to interference of the environment with the measurement process . The concept of random error is closely related to the concept of precision . The higher the precision of a measurement instrument, the smaller the variability (standard deviation) of the fluctuations in its readings . </P>

Errors that consistently cause measurement value to be too large or too small