I've read the following excerpt from a book (Passive Circuit Analysis with LTSpice by Colin May page 96) but am not sure what that means (with no relevant explanation before nor after):
On Fixed Resistors: a useful point to consider if good stability and reliability are essential is to derate the resistor so that its temperature is far less than the operating maximum. Thus, if the actual power is 1/10W, instead of a 1/8W resistor, use 1/2W.
Resistance of a resistor increases with temperature. So if the temperature is far less than the operating maximum, we're lowering the specified resistance of the resistor, and as a result the resistor (under lower temperature) will load itself with a higher current. Under such condition (same voltage and higher current) the resistor dissipates more power - hence it makes sense to choose a resistor with higher Wattage rating (1/2W instead of 1/10W). I don't see any meaning to it other than reminding us to change higher rated resistor when temperature gone low.
How does choosing a resistor with a higher rating (1/2W) has anything to do with testing the stability or reliability of a circuit? And why is it considered "derating" when you switch to 1/2W resistor as you lower the temperature?