HI
On 7 Dec 1999, Magill, Brett wrote:
> I am a graduate student in sociology studying individual's perceptions of
> control (locus of control) using existing data. The data set include four
> items to measure this construct which were taken from a larger scale of more
> than twenty, the larger scale reaching an acceptable level of reliability (I
> do not know the exact level, but it is a widely researched and used
> instrument) in previous research. The four items that were included were
> selected as the best measures of the construct based on empirical evidence
> (item-total correlation's, factor analysis).
>
> In my own research, I used these items and decided to sum responses across
> these four likert-type items. However, the Alpha reliability is very low
> 0.30 (items were reverse scored as necessary and coding was double-checked).
> I defended the decision to sum the items, despite the low Alpha, based on
> the fact that they were selected from a larger set of items which are
> internally consistent. In presenting my findings, I was heavily criticized
> for this decision.
One thing to point out to critics (and to those who argued
against summating in response to your question) is that the
reliability of a single-item is generally going to be lower than
the reliability of a multi-item scale. I would determine the
theoretically expected reliability of a 4-item scale, as
suggested by another poster, and also do a few other things.
Examine the Alpha output to see whether one of the items is
causing particular problems. It may be that some set of 3 items
has notably higher reliability. Do a factor analysis of the four
items and see whether a single factor emerges and/or whether the
pre-rotated loadings on the first factor are all positive.
But you might also want to explore the possibility that indeed
the 4 items do measure different things and have different
relationships with your predictor variables. Can you easily
repeat your analyses four times and do the results largely
duplicate one another? If so, this would be another rationale
for aggregating the items together and reporting a single
analysis. Or if you factor analyze the 4 items along with your
predictor variables, do the 4 items (or some subset) of them all
load on the same factors?
Best wishes
Jim
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James M. Clark (204) 786-9757
Department of Psychology (204) 774-4134 Fax
University of Winnipeg 4L05D
Winnipeg, Manitoba R3B 2E9 [EMAIL PROTECTED]
CANADA http://www.uwinnipeg.ca/~clark
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