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Indexes and scales

Indexes and scales both combine several items into one number, for opposite reasons. In an index, the items cause the score: assets, each contributing independently, so dropping one changes what the index means. In a scale, the score causes the items: an underlying trait expressing itself through responses that should correlate because they share a source. The direction of that arrow decides what you may do with the number — whether internal consistency is evidence of quality or beside the point, and whether items are interchangeable.

Also covered: indexes · scales · latent variables · formative vs reflective measurement