Objectivity is a cornerstone of all measurement. It requires that agent calibrations (e.g. item difficulties) be independent of the sample of objects (e.g. persons) used in the calibration procedure. Object measures must also be independent of the particular agents used to obtain them. Thurstone (1928) stated: "...the scale values of the statements of opinion must be as free as possible, and preferably entirely free, from the actual opinion of individuals or groups." As Wright & Stone (1979) remark, Loevinger (1947) offered a similar formulation as a criterion for absolute scaling, but it was Rasch (1961) who made what he called "specific objectivity" the central requirement of a new approach to measurement.
Specific objectivity requires that differences between pairs of object measures or pairs of agent calibrations are sample independent. This means that two agents must be found to differ by the same (i.e. a statistically equivalent) number of measurement units no matter what sample of objects actually responds to the agents. Similarly, two objects must be found to differ by the same number of units no matter what sample of agents (from the universe of relevant agents) is used in the measurement procedure. In other words, the relative locations of pairs of objects and pairs of agents on the underlying continuum must be sample independent.
General objectivity, essentially attained by measures in physics and chemistry (e.g. thermometers), requires that the absolute location of an object on, say, the Celsius scale, is sample independent. Temperature theory is well enough developed that routine manufacture of thermometers occurs without even checking the calibrations against objects with known values prior to shipping the instruments to customers. Such is our collective confidence in temperature theory. We know enough about liquid expansion coefficients, the gas laws, glass conductivity and fluid viscosity to construct a remarkably precise measurement with recourse only to theory. Measurement of the temperature of two objects results in not just sample independence for the difference between their temperatures, but sample independence for the point estimate of each object's temperature reading. It does not matter what thermometer we use, or how it was constructed, the Celsius value will be the same.
The difference between specific and general objectivity is seen not to be a consequence of the fundamental natures of the social and physical sciences, nor to be a necessary outcome of the method of making observations, but to be entirely a matter of the level of sophistication of the theory underlying the construction of the particular measurement instruments.
Note: For our early attempt to relate Cronbach and Meehl and Messick's later integration to the Rasch model see Stenner,A.J., Smith,M. and Burdick, D.S. " Toward a theory of construct definition" Journal of Educational Measurement, 1983,20,4, 305-316. It was in part an interest in the relationships between construct validity and Rasch measurement that led to the Lexile Framework for Reading.
Objectivity: Specific and General, J Stenner Rasch Measurement Transactions, 1990, 4:3 p. 111
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