"A major contribution of G-theory is that it permits a decision maker to pinpoint the sources of measurement error and change the appropriate number of observations accordingly in order to obtain a certain level of generalizability" (Marcoulides 1993 p. 197).
Sources of measurement error are identified and quantified in a Generalizability Study (G-study). Decisions are then made concerning which of these sources are small enough to be ignored or, better, which sources permit a reduction in the number of relevant observations in the subsequent Decision Study (D-study) without significantly reducing the generalizability coefficient (i.e., reliability).
Since resources are invariably limited, such information could be helpful in evaluating how best to employ those resources. Marcoulides points out that these decisions have often focussed on reliability and measurement error exclusively, ignoring sampling error and sample size considerations. He suggests how both types of error can be addressed simultaneously in meeting budget constraints.
Consider a Marcoulides' formula. It suggests a simple solution to a typical design problem. When one wants to minimize error variance for a paper-and-pencil test of L items, administered to N subjects, with cost-per-response of C and a budget of B, then N and L are given by
where Vp is the "true" variance of the person responses across items, and Vi is the "true" variance of the item responses across persons, and Vi and Vp are estimated during the G-study. How well these values of Vi and Vp will carry forward to the D-study, however, is never determined.
In Many-facet Rasch Measurement (MFRM), a preliminary study, i.e., a G-study, is not necessary. When the items are reasonably targeted on the persons, measurement error can be estimated from theory, e.g., via a nomograph (RMT 1991 7:3 283). The measure variance of the subjects and test items is usually well enough known from experience. If an earlier data analysis of similar data is available, the results of that analysis can be used.
In MFRM, Marcoulides' formula carries forward directly. The algebraic form is identical, but now Vp is the adjusted, i.e., "true", person measure variance, and Vi is the adjusted, i.e., "true", item measure variance.
MFRM, however, goes beyond G-theory in providing a comprehensive framework for test design. G-theory starts just before the first data collection, the G-study. MFRM starts with the first conceptualization of the testing problem. What is the variable on which measurements are to be obtained? How is this variable to be expressed in terms of an ascending sequence of test items? How is the rating scale to be defined to accord harmoniously and unambiguously with the variable definition?
G-theory stops before the main data collection starts. At this point all error terms have been quantified, and will be asserted to maintain their values during the ensuing data collection. MFRM continues until the test reports have been written, and will continue to influence the readers as long as they maintain interest in that measurement variable. In particular, MFRM continues to estimate measure variances and measurement error throughout the data analysis. It also evaluates and reports data quality (fit), identifying items and persons with performances meriting special attention, perhaps remediation, perhaps even omission from the current analysis. When data analysis starts while data collection is still underway, MFRM monitors data and judging plan quality.
There is, however, yet a further fundamental issue raised by differences between MFRM and G-theory. The ultimate goal of G-theory is "reliable" measures, generally expressed in the reproducibility of raw scores. There is a point, however, at which such raw score reliability is achieved at the expense of validity (See Engelhard, RMT 6:4 257 on the attenuation paradox). This is particularly true in the case of judge-mediated examinations in which almost all the recommendations on how to achieve "reliable scores" involve restricting how judges grade, and/or atomizing the tested behavior into some form of unrecognizable check list.
In MFRM, although the statistical bases for reliable measures are still present, the emphasis shifts to the reproducibility of measures of a students' ability. These measures are freed from the particular items used to elicit this performance and the particular judges that rated the performance, so reproducibility of raw scores is of little importance. Even measure reproducibility is seen to have its limits. No student (or item, or judge) functions consistently at exactly the same level of performance throughout the entire testing experience. Consequently, the endeavor to estimate measures with pin-point precision is futile. Usefully stable measures are the best that can be hoped for, and, in fact, all that is needed.
By focusing on the reproducibility of the latent trait rather than forcing judges to generate the same raw score, the concept of reliability takes its rightful place in supporting validity rather than being in tension with it. This I consider to be the crucial point that differentiates MFRM from G-Theory.
John A. Stahl
Marcoulides, George A. 1993. Maximizing Power in Generalizability Studies Under Budget Constraints, Journal of Educational Statistics, 18:2 197-206.
"Generalizability Theory" in Facets Help
"Construction of Measures from Many-facet Data" pp.495ff. John M. Linacre. Benjamin D. Wright. Journal of Applied Measurement (2002) 3(4), 484-509.
What Does Generalizability Theory (G-Theory) offer that Many-Facet Rasch Measurement cannot duplicate? Stahl JA. Rasch Measurement Transactions, 1994, 8:1 p.342-3
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