The local resistance inside the resistor varies substantially and the resistor may be viewed as a complicated network of series and parallel resistors, each acting as a noise source. This and other factors make carbon composition resistors very noisy when compared with metal film and wire-wound resistors. This is the deviation from the resistor's nominal value, expressed as a percentage. It is measured at 25 C with no load applied. At temperatures above this, the maximum allowable dissipation must be derated.
Manufactures often give a simple linear curve, similar to the one below.
The area below and to the left of the curve is the valid region of operation. This is the change in resistance with applied voltage. It is normally associated with carbon composition and carbon film resistors. High value resistors tend to have high voltage coefficients. Metal, oxide, metal film, and wire-wound resistor are typically 10 times more stable.