Omega

Reliability is concerned with the consistency of scores on a measure when the measure is administered to the same group of individuals under comparable circumstances. Scores on a measure are often computed as the sum of scores across items in [Page 1177]the measure and are referred to as scale scores. Reliability of scale scores is defined in classical test theory as the ratio of scale true score variance to observed scale score variance. It can also be defined as the correlation between scale scores and the scores from its parallel form, or the square of correlation between scale true scores and observed scores. That is, ${\rho }_{X{X}^{\prime }}={\sigma }_T^2/{\sigma }_X^2=1-{\sigma }_E^2/{\sigma }_X^2$. The population ρXX′ is unknown in practice and needs to be estimated from data.

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