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Encyclopedia of Measurement and Statistics

Neil J. Salkind & Kristin Rasmussen

Pub. date: 2007 | DOI: http://dx.doi.org/10.4135/9781412952644

Print ISBN: 9781412916110 | Online ISBN: 9781412952644

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Encyclopedia Entry

Parallel Forms Reliability

Michael C. Rodriguez

All observations and all measurements contain error. The focus of much work in measurement is to minimize and estimate the amount of error in any given measurement. In classical test theory, X is an observed score that is composed of T , the true score, and E , the error score: X = T + E. The true score is never known, but can be thought of as the long-range average of scores from a single instrument administered to an individual an infinite number of times (the expected value or expected score). The error score is random and may have many sources, including testing conditions, individual characteristics that fluctuate from administration to administration, differences in forms, or instability of an individual's ability or trait over time. This random error score is quite different from systematic sources of error, like testwiseness, which may systematically increase an individual's score on each administration. ...

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