Wundt asked people to make judgments about "psychophysical phenomenon" -- about weights for example, he would say, "Does this weigh more than this?" and point at two weights. He was the first one to try to measure things of the mind. Thurstone measured attitude and achievement. In these examples there is some error in judgment on the part of the participant. Some people are better at making judgments about weights than others. The same is true for the "strength" of an attitude, emotion or achievement. Psychological measurements (in fact all measurements) contain error and consequently our assessments and the mathematical models (statistics) must make provisions for such error. Psychology is not at the level of measurement of other sciences. For example, other sciences have "scopes"; telescopes, the microscopes, stethoscopes, and the sphygmomanometers. The measurement of personality and intellectual attributes has been harder to come by--we have no scopes.