In comparing measurement theories, it is evident that the awareness of the concept of measurement error during the time of Galileo has lead to the formulation of observed scores comprising a true score and error (classical theory), universe score and various random error components (generalizability theory), or individual latent ability and error estimates (latent trait theory). The definition of a true score and the definition of measurement error separates measurement theories. Students who need practical applications can progress through the traditional software for each of these theories. Using the software for the various approaches will give students an understanding of the measurement theories, scaling, objective measurement, and the different definitions of true score and error. (Contains 2 tables and 21 references.) (SLD)