Stage models. Innovation is treated principally as a sequential linear activity; that is, the process of innovation is viewed as a series of discrete stages. The unit of analysis within each stage is generally activity or functional responsibility. Examples are offered by Utterback (1971), Manstield and Wagner (1975), and Saren (1984). Although useful, these models overlook the nonlinearity of the innovation process. Kelly and Krantzberg (1978) have drawn attention to the poor representation that these models provide of the cornplexity of the innovation process.