[Churgin, J. (1985). How to Count the Uncountable. Warsaw: Common Knowledge.]Search in Google Scholar
[Encyclopedia (2004). Kraków: Polish Scientific Publishers PWN, Vol. 10.]Search in Google Scholar
[Stevens, S.S. (1946). On the Theory of Measurement Scales. Science, Vol. 103, No. 2684.10.1126/science.103.2684.677]Search in Google Scholar
[Wiśniewski, J.W. (1986). Econometric Study of Qualitative Phenomena. Methodological Study, Toruń: Scientific Publishers, Nicolaus Copernicus University.]Search in Google Scholar
[Wiśniewski, J.W. (2009). Microeconometrics. Toruń: Scientific Publishers, Nicolaus Copernicus University.]Search in Google Scholar
