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Scales and Indexes
  • What's the difference between a scale and an index?
    • The terms are used imprecisely and interchangebly in social research
      • Typically, both scales and indices employ multiple observations or items of measurement
      • However, an index usually combines observations without concern about their intercorrelations:
        • Dow Jones Industrial Index (DJIA) of 30 large U.S. stocks
        • Consumer Price Index (CPI) of 73 components
      • A scale often evaluates item-intercorrelations before selecting items for inclusion.
  • Sometimes, the distinction hinges on whether the indicators measure cause or effect.
Indicators "cause" the concept
  • An index tends to measure a concept that depends on what happens in the real world.
    • It is "built-up" from a set of causes, e.g., DJIA and CPI
  • The multiple indicators may be considered "compensatory"
    • --a high score on one indicator may compensate for a low score on another
    • --so they don't have to intercorrelate to make meaningful contributions
Indicators reflect the "effects" of the concept
  • A scale tends to measure a concept that is "latent" in the real world--e.g., party identification, prejudice, intelligence.
    • One "taps" into the concept by measuring its effects
  • The multiple indicators may be considered "alternative"
    • --although each indicator is imperfect (due to error), each is regarded as an equally imperfect alternative
    • --they must be intercorrelated to support this assumption.