A correction factor (CF) can be determined if the error is known. It is the quantity that should be added to the observed value in order to correct it to the true value. As a mathematical expression,

Rearranging this, OV ? TV = ? CF.

Recalling that the expression for the error was ? = OV ? TV, then the correction factor CF must be equal to the negative of the error. Thus, if there is a scale that is showing a weight of 0.5 kg when it should be reading zero, then the correction factor for any reading taken from this scale should be ?0.5 kg.