“…The latter issue has been investigated extensively (for reviews, see Birnbaum, 1982Birnbaum, , 1990, but with mixed results. Whereas for most continua (such as loudness or heaviness), observers seem to employ a single operation whether instructed to judge differences or ratios (e.g., Mellers, Davis, & Birnbaum, 1984;Schneider, Parker, Farrell, & Kanow, 1976), there are occasional reports ofdifferent operations' being employed under these instructions (e.g., Birnbaum, Anderson, & Hynan, 1989;Popper, Parker, & Galanter, 1986). Narens's conceptualization, especially the inherent distinction between numerals and numbers, may shed new light on the issue of comparing direct ratio estimation with difference estima-tion both by providing a thorough theoretical basis (the foundations of which are developed in Narens, 1997) and by suggesting new methodologies to tackle the problem.…”