The parts will be first counted. Information to component stressing and environment also becomes very important. Each component’s failure rate li is expressed as a base failure lb and as a series of multiplicative correction factors:

li = lb PE PQ PA … PN
lb = base failure rate at a specified temperature and primary stresses (voltage/current either or both) normal/shear stresses
PE = factor related to environment (vibration, humidity)
PQ = level of specification under which the component has been produced and tested
PA = applications factor etc., up to N factors

In the graph the failure rate versus temperature for different levels of applied stress (power, voltage, etc.) is indicated. The stress level 1 the single star curve is showing how the generic failure rate is increasing with temperature. Similarly the double star, the triple star are a two different stress levels. So the generic failure rate is the main reference.