Measurement errors can be divided into systematic errors, random errors, and errors according to their causes, occurrence rules, and their effects on the measurement results.

(1) System error: Under the specified conditions, the error that the symbol remains unchanged or changes according to a certain rule, the boxing production line is called the system error. The system error in which the sign is constant is the fixed value system error, and the system error that changes regularly is the variable value system error. Such as the error of the gauge block, the error of the ruler, and the error of the eccentricity of the dial. Most of the systematic errors can be corrected or after finding out the changing law.

(2) Random error: Under specified conditions, the error in which the symbol changes in an unpredictable manner is called random error. As far as a certain measurement is concerned, the occurrence of random errors is irregular and cannot be followed. However, if multiple repeated measurements with equal precision are carried out, it has the same basic characteristics of statistics as other random, and the range of random error value can be estimated through analysis. Random errors are mainly caused by random factors such as temperature fluctuations, changes in measuring force, instability of the transmission mechanism of measuring instruments, parallax, etc. Although they cannot be, as long as the causes are carefully and carefully analyzed, their influence on the measurement results can be reduced.