Reliability Concepts
– Definition of Reliability
– Test consistency
– Classical Test Theory
– X = T + E
Obtained True Random
Score Score Error
– Estimation of Reliability
– Correlation coefficients (r) are used to estimate reliability
– The proportion of variance attributable to individual differences
– Directly interpretable
– Reliability of .90 accounts for what % of the variance?
• Conceptual True Score Variance
– Fine for true score variance
– T = X - E
– Convert formula to variances (σ2)
– Partial out the ratio of obtained score variance and error variance
– Substitute the ratio of obtained score variance for 1
– Reliability is the ratio of error variance to obtained score variance subtracted from 1
– Types of Reliability
– Test-retest
– Alternate Forms
– Split-Half
– Inter-item consistency
– Interscorer
– Test-Retest Reliability
– Coefficient of stability is the correlation of the two sets of test scores
• Alternate (Equivalent)
Forms Reliability
– Coefficient of equivalence is the correlation of the two sets of test scores
– Split-half Reliability
– Coefficient of internal consistency is the correlation of the two equal halves of the test.
– Reliability tends to decrease when test length decreases
– Spearman-Brown Formula
– A correction estimate The S-B formula is computed with the following ratio:
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