This article brings back memories of school days and some of those courses we have no option but to sit through it :)

Reliability of manufactured product is of utmost importance, what I do is to add mine after the specification recommended theirs- e.g. if a light bulb is said to last for 10000 hours, I just assume it'd last for 70% of that time or 7000 hours. That way you'd never be disappointed :)