The operating and dormant phases of use must be identified because device failure rates differ widely depending on whether an item is active or inactive, and whether it is likely to be affected by the frequency of switching on and off. Also, when an item is operating, its failure rate will depend upon the ratio of the applied stress to the design (or rated) stress; therefore, the precise operating conditions must be defined whenever a detailed parts stress analysis is to be carried out. For additional information, refer to Parts Stress Analysis Methods.