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Topic: _pragmatic_ item/scale analysis
Replies: 3   Last Post: Jun 20, 2004 10:40 PM

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Jim Clark

Posts: 79
Registered: 12/6/04
Re: _pragmatic_ item/scale analysis
Posted: Jun 20, 2004 1:47 PM
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On Sun, 20 Jun 2004, PD Dr. Hans-Christian Waldmann wrote:
> A client to our university stats consulting has produced a
> questionaire with over 250 items that he considers to be
> associated to 12 scales (with 6-30 items for a scale). There
> is consent that the number of items is far too high in a
> clinical context, and has asked us to reduce the item count
> in a "pragmatic" way. Hm. He objects to the idea of re-
> evaluating content validity ("they're all reasonable").
> Nevertheless, I've been asked to cut 30% of the items while
> retaining the 12 scales.

You do not say whether you have pilot info on responses to all
items, or how many subjects you have, if so. You might want to
consider not only the homogeneity of the items (i.e., convergent
validity), which your criteria primarily address, but also the
discriminant validity (i.e., whether items correlate with other
scales). One approach, given enough subjects, would be factor
analysis and select items that load purely on the desired
factors (i.e., loadings on only one scale). At least, items
should correlate more highly with scale they are assigned to than
to other scales.

Another consideration in scale creation (see Doug Jackson's work
on personality, vocational, and clinical assessment) is
correlation with various measures of response bias (e.g., Paulhus
or other measures, desirability). It might be good, for example,
if half the items on each scale were positively worded and half
negatively worded.

Without knowing the domain, 12 seems like a lot of factors to try
and get independent measures of, so results are likely to be
pretty messy.

Best wishes

James M. Clark (204) 786-9757
Department of Psychology (204) 774-4134 Fax
University of Winnipeg 4L05D
Winnipeg, Manitoba R3B 2E9

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