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Factor analysis and reliability

Factor analysis asks whether they measure one thing or several — whether the correlations are consistent with a single underlying trait or betray two. Reliability then asks whether they measure it consistently, and it has more than one meaning: test-retest over time, internal consistency across items, inter-rater agreement between observers. A reliable instrument can still be invalid, measuring the wrong thing precisely. Reliability is necessary and nowhere near sufficient.

Also covered: factor analysis · test-retest reliability · internal consistency · inter-rater agreement