False calls are indications that are classified as anomalies where no imperfection, defect, or critical defect exists. MFL tools, by their nature, receive signals from pipeline features and non-relevant conditions. Occasionally, these indications are characterized as anomalies. Two common causes of false calls are metal objects near the pipeline and sleeve eccentricities. If these features are reported as imperfections or defects, costly excavations and remedial work may be performed, where none is needed.

False calls can lead to a significant amount of additional work if the cause of the false call cannot be located, particularly on a line with many indications. When no anomaly is observed on the outer pipe surface, an anomaly may be present on the inner pipe surface. Additional inspections using other nondestructive techniques are needed to look for internal anomalies.

An acceptable level of false calls depends on how the inspection results are used. If all indications that are characterized as anomalies are excavated, the number of false calls must be low. There are few published reports of acceptable false call rates. Some operators specify "none to be classified as significant defects" while other investigate all indications.

Missed calls are a far more serious concern than false calls, especially if the anamoly could threaten the integrity of the pipeline. Missed calls result from errors in data analysis or from blind areas. Errors in analysis usually result from anomalies that produce small-amplitude signals even though the anomaly is large or severe. Analysis errors also result when signals from anomalies are mistaken for those from pipeline features or benign conditions.