The error with obvious distortion measurement result is called gross error. This error is caused by the operator's careless (such as misreading, mismembering and miscalculating the data etc.), in correctively operation, suddenly changing of experimental conditions, or hurry experiment when the experimental conditions has yet to reach expected requirements. The measured value containing gross error is called abnormal values and bad values. Generally speaking, all the bad values shall be eliminated from the test results, but for the suspicious measurements with unknown causes, they shall be judged according to a certain criteria, then it can be decided whether these bad values shall be eliminated from the measured results.