[1] For the theory of extensive measurement, see Krantz, D. H., Luce, R. D., Suppes, P., & Tversky, A. (1971), Foundations of measurement, Vol. I. San Diego: Academic Press. Seminal works on semiorders are due to Luce, R.D. (1956), “Semiorders and a theory of utility discrimination”, Econometrica and on interval orders to Fishburn, P.C., (1973), “Interval representations for interval orders and semiorders”, Journal of Mathematical Psychology. and have an economic interpretation with the idea of “thick indifference curves”.