Ivlev's electivity index

electivity index (an index (E) showing the degree of choice a predator shows in its feeding behaviour, e.g. of a predator on plankton; E is derived from r_i - p_i / r_i + p_i where r_i is percent composition of plankter i in the gut contents and p_i is percent composition of plankter i in the plankton sample. Values for E range from -1 to +1 with -1 meaning complete avoidance, 0 meaning no active selection, and +1 meaning complete selection)