The physical quantity of an object being measured with a certain value is called the real value, and the difference between the measured value and the true value is called error. According to the causes of errors, they are usually divided into two categories, namely, systematic error and accidental error.
        System error is the error caused by the fixed reason, in the determination process according to certain rules, repeated, a party that was always gay, high or low always, the magnitude of the error is measured, so it is also called "measurement error". It comes from the analysis method error, instrument error, reagent error and subjective error, for example, the analyst grasps the operation rule and the operation condition and so on.
        The accidental error is due to some accidental error caused by external factors, the cause is often not fixed, unknown, and sizes, either positive or negative, its size is not observed, such sources of error are sometimes difficult to perceive, may be due to the environment (pressure, temperature, humidity) the accidental fluctuation or the performance of the instrument, analysis personnel inconsistency of the samples during processing.

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