In classical test theory the observed score on a measure (Y) can be partitioned into two parts: true score (T) and error (e). In symbols, this is expressed as Y = T + e. T can be defined as the mean of a very large number of independent measurements. e is assumed to be random and independent of the value of the true score. The reliability coefficient represents the proportion of the variance in the observed Y scores (a2y) that is true score variance ((72T),
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