There are two main classification systems in the social sciences for measuring different types of variable. One was developed by S. S. Stevens (in 1946; see Stevens 1951) and the other by Duncan (1984). According to Smith (1991: 72), ‘Duncan (1984) has enumerated, in increasing order of interest to scientists, five types of measurement: nominal classification, ordinal scaling, cardinal scaling, ratio scaling, and probability scaling'. Duncan (1984: viii) writes about Stevens's classification as follows