Validity stance regards the overall score by the scale as if it were the external criterion validity is interested in that item-to-total correlation, rather than in how well items duplicate each other. The lesser is the error the more item-item homogeneous is the scale. Reliability stance believes that the measured trait is simple, unifactor structure each item measures it and just it + is liable to some random error. They seem to be two different (and perhaps competitive) paradigms about correlations observed among the items constituting the scale. As seen for example with the term "internal consistency"$^1$. Often seen as two different facets of an instrument functioning, reliability and validity sometimes are difficult to separate conceptually from each other. Reliability of a psychometric scale is its correlation with its own "true" score, and uncorrelation with extraneous random factors. Validity of a psychometric scale is its correlation with the trait supposed being measured (called criterion), and uncorrelation with extraneous systematic factors.
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