The main principle in the measurement of opinion is the construction of a rational base line for describing the distribution of opinion, on which equal intervals shall represent equally often noticed shifts in opinion or equal appearing opinion differences. This principle of a rational scale enables one to compare several groups as to distribution, central tendency, and dispersion of opinion on any stated opinion variable, irrespective of the shape of that distribution or the amount and direction of bias in each of the groups. (PsycINFO Database Record (c) 2006 APA, all rights reserved)