Measuring instruments are mechanical/electrical devices. Their accuracy will sometimes change with use, and is guaranteed to change with abuse. Checking the accuracy of instruments is called calibration. Instruments are calibrated by checking their performance against a standard. The measured difference in the length or angle between the averaged measurements and the standard, is error. This error can be a random error or a systematic error. Random error is unpredictable and must be controlled by following recommended techniques and procedures. Systematic errors are usually caused by damage or wear to the instrument. If the calibration process indicates a systematic error, an error that is constant with each measurement, then a correction factor can be calculated so that each measurement taken with the instrument can be corrected.
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