The second step is the diagnostic where an abnormal situation is investigated to establish the type of fault, the severity of the problem and the corrective actions to be taken. The diagnostic step needs to be carried out only on those units that are deemed “abnormal”(usually less than 10% of the population). Tests that would be applied can be more expensive and off-line, and they need to be focused on individual attributes in an attempt to arrive at an unambiguous diagnosis.