Fractional designs are expressed using the notation "l"k − p, where "l" is the number of levels of each factor investigated, "k" is the number of factors investigated, and "p" describes the size of the fraction of the full factorial used. Formally, "p" is the number of "generators", assignments as to which effects or interactions are "confounded", "i.e.", cannot be estimated independently of each other (see below). A design with "p" such generators is a 1/("lp")="l-p" fraction of the full factorial design.