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Scales
and Indexes
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- 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.
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Indicators "cause" the concept
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- 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
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Indicators reflect the "effects" of the
concept
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- 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.
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