1. Co-citation is far from being the only method used to measure science. For an indication of the range of possibilities see, e.g., Elkana et al. (1978), Ellis, Hepburn and Oppenheim (1978), Freeman (1983), Garfield (1979), Gilbert and Woolgar (1974), Machlup (1972), Menard (1971), Narin et al. (1976), National Science Board, Science Indicators (1973, 1975, 1977, 1979, 1981), OECD (1982), Pavitt (1978, 1982), Schmookler (1966), Small (1977), Price (1963) and the journal Scientometrics.