• Carelessness on the part of observer in taking or recording reading;
• Faults in equipments;
• Adoption of wrong technique.
• Misinterpretation.

The blunders or mistakes result into large errors and thus can easily be detected by comparing with other types of errors (generally small in value). The maximum permissible error in an observation is ± 3.29 s (where s is the standard deviation of sample distribution) and is used to separate mistakes or blunders from the random errors. If any error deviates from the mean by more than the maximum permissible error, it is considered as a gross error and the measurement is rejected.

After mistakes have been detected and eliminated from the measurements, the remaining errors are usually classified either as systematic or random error depending on the characteristics of errors.

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