Science. 1963 May 3;140(3566):453-60 doi:10.1126/science.140.3566.453

Fager EW, McGowan JA.

Zooplankton Species Groups in the North Pacific

Fager & McGowan index of affinity: [J/√(Na.Nb)] - 1/2√Nb, where J is number of joined occurences and Na and Nb are the number of occurences of a and b respectively. Nb is chosen to be greater or equal to Na. Scores higher than 0.5 denote affinity. The paper then explores the geographical distribution of groups of species, where all species in the same group have affinity for each other.