Generalizability Theory

Noreen M. Webb & Richard J. Shavelson

Generalizability (G) theory is a statistical theory for evaluating the dependability (“reliability”) of behavioral measurements. G theory pinpoints the sources of systematic and unsystematic measurement error, disentangles them, and then estimates their magnitudes simultaneously. In G theory, a behavioral measurement (e.g., an academic self-concept score) is conceived of as a sample from a universe of admissible observations . This universe consists of all possible observations that decision makers consider to be acceptable substitutes (e.g., scores sampled on Occasions 2 and 3) for the observation in hand (scores on Occasion 1). Each characteristic of the measurement situation (e.g., survey form, item, occasion, rater) is a potential source of error and is called a facet of a measurement. The universe of admissible observations is defined by all possible combinations of the levels (called conditions ) of the facets. To evaluate the dependability of behavioral measurements, a generalizability (G) study is designed For ...

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