Reliability refers to the ability of an instrument to consistently measure a construct, and must be empirically demonstrated in order to make an argument for internal validity. As described by William Trochim (2000), measurement reliability refers to the consistency and stability of a measure and is estimated based on the proportion of variability in the measure attributable to the true score. Said differently, a test, scale, or other measurement tool is considered reliable if it can get the same score repeatedly—assuming no change is expected. Reliability is estimated through four general classes of reliability estimates, including interrater or interobserver reliability; test–retest reliability; parallel-forms reliability; and internal consistency reliability. One type of internal consistency reliability is split-half reliability. In this entry, this type of reliability is ...

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