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Estimating Test Reliability

Lord, Frederic M.
Publication Year:
Report Number:
ETS Research Bulletin
Document Type:
Subject/Key Words:
Error of Measurement, Reliability Formulas, Test Reliability


Formulas for parallel-form reliability coefficients are derived from two improved definitions of parallelism. One definition, based on randomly parallel test forms, leads to a new and useful derivation for the KR formula-21 coefficient. The other, based on matched test forms, leads to a formula for the least upper bound of the test reliability. A numerical example is given, showing how a standard error of measurement for each separate examinee is readily computed, and how the test reliability may be computed by averaging these standard errors.

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