The Nelson complexity index (NCI) describes a measure of the secondary conversion capacity of a petroleum refinery relative to the primary distillation capacity. It was developed by Wilbur L. Nelson in a series of articles that appeared in the Oil & Gas Journal from 1960 to 1961 (Mar. 14, p. 189; Sept. 26, p. 216; and June 19, p. 109). In 1976, he elaborated on the concept in another series of articles, again in the Oil & Gas Journal (Sept. 13, p. 81; Sept. 20, p. 202; and Sept. 27, p. 83).