Life of a Component at half Load
MIL-HDBK-217F is a masterful source on reliability despite its longevity. For example, you can choose a resistor, capacitor, diode' transistor or IC, place it in the category that best describes its circuit limits (and environmental factors) and get a meaningful value to its MTBF (mean time between failures).

I still use it today (when the customer asks (and they still do)) and I don't know of any more reliable source.

You can choose to to set its operating parameters to 50% if that is what the target circuit runs at and you can get real MTBF numbers out. Don't be dissapointed though; this document is not an easy read for a beginner. Be prepared to consider what sort of operating environment the component is used in. Here you might be unfamiliar with terms such as ground-benign or ground-mobile or, at the very extremes,canon shot (i.e. missile).

It's a good document and still used today despite its age.