Derived statistics or secondary statistics in the strict sense are attributes of a character in the SPECIAL system which are based on (or derived from) the character's primary statistics. More commonly, the term refers to all character attributes which only exist as a function of another value, i.e. character attributes which the player cannot determine or influence directly.
Most derived statistics are directly based on a primary statistic, but may also be altered independently, for example by perks and traits. Carry Weight, for instance, is directly based on strength, but can be lowered by the Small Frame trait or increased by the Strong Back perk.