I have never said that split half is better in general. In general, alpha
is probably better. But in those instances when alpha doesn't fit, the
split half technique may be quite useful. Coefficent alpha is based upon a
model like all of statistics. If the model is inappropriate, then some
other model should be used. Of course, we could use alternate forms or
test-retest but they have obvious shortcomings as well. The problem with
using alpha when it is not appropriate is that, being an average, it
doesn't tell you the range. We all know that the mean is only half the
story. A random split compared to alpha gives you more information than
either one alone.
At 03:53 PM 4/18/00 -0400, you wrote:
>Trying to have it both ways,
>on 18 Apr 2000 08:13:08 -0700, [EMAIL PROTECTED] (Paul R
>Swank) wrote:
>
>> Depends on whether you consider a lack of item homogeneity as unreliability
>> or not. If your content is supposed to be homogeneous then a lack of
>> homogeneity implies your test has problems. If your content is not
>> necessarily homogeneous then the reduced alpha appears to say that your
>> test is unreliable when that's not actually the case. All reliability
>> coefficients suffer from the same problem. They are all sensitive to a lack
>> of reliability and something else. Test retest is sensitive to trait
>> instability, alternate forms is sensitive to a lack of parallelness, split
>> half is sensitive to the bad split, and alpha is sensitive to lack of item
>> homogeneity. It's like any statistical model. If the model isn't
>> appropriate, the result is misleading.
>>
>So, Paul S. is claiming that split-half is better because it is not
>precise? Or, are we supposed to carefully select the two halves so
>they will match, for our split-half reliability? -- that is the one
>strategy that might be unbeatable, if you really have a directions on
>how to split the scale. But Paul does not say that much, so far as I
>can tell. If you don't do that, it seems that it *ought* to be
>impossible to favor one accidental split-half, compared to,
>"Using alpha, which gives an average of all split-half results."
>
>By the way, the standardized item-alpha (provided, for instance, by
>SPSS Reliability procedure) is computed STRICTLY from the
>correlations, with no reference at all to variances. Usually, that
>should be an optimistic estimate.
>
>Paul Gardner laid out most of the relevant questions, quite well.
>
>--
>Rich Ulrich, [EMAIL PROTECTED]
>http://www.pitt.edu/~wpilib/index.html
>
------------------------------------
Paul R. Swank, PhD.
Advanced Quantitative Methodologist
UT-Houston School of Nursing
Center for Nursing Research
Phone (713)500-2031
Fax (713) 500-2033
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