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Coefficient alpha, or Cronbach's alpha, is one way to quantify reliability and represents the proportion of observed score variance that is true score variance. Reliability is a property of a test that is derived from true scores, observed scores, and measurement error. Scores or values that are obtained from the measurement of some attribute or characteristic of a person (e.g., level of intelligence, preference for types of foods, spelling achievement, body length) are referred to as observed scores. In contrast, true scores are the scores one would obtain if these characteristics were measured without any random error. For example, every time you go to the doctor, the nurse measures your height. That is the observed height or observed “score” by that particular nurse. You return ...

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