Re: split half reliability

[Paul R Swank]

At 12:55 PM 4/19/00 +1000, you wrote:
>Paul R Swank wrote:
>>
>> High alpha can be obtained when not all items are highly intercorrelated
>> with all the other items but it requires having enough items. Lack of item
>> homogeneity will certainly be greater problem with short scales. With
>> respect to the Spearman-Brown, I don't recommend it. I prefer the
>> Guttman-Flanagan split half which is just a special case of alpha for two
>> items.
>>
>I would agree with everything you've said here, but I still would press
>you to explain what, exactly, you mean, when you use the term
>"homogeneity".

Item homogeneity means essentially tau equivalent items. That is, each item has expected value of tau plus a constant. Thus, they are all measuring the same construct.

>I'm not familiar with the Guttmann-Flanagan formula, and I'd be pleased
>if you would describe it. From your description, it sounds like the
>following:
>rel = 1 - H/T
>where H = sum of the variances of the two split half scores
>T = variance of total test scores.
>Is that correct?

Exactly. The Guttman-Flanagan split half requires the halves to be essentially tau equivalent rather than parallel as the Spearman-Brown correction requires.
>
>The question I raised earlier about how one splits the scale into halves
>still remains. And your response raises another question: why do you
>prefer G-F to Sp-Brown? What is the criterion for "better" here?

Because of its reduced assumptions (essentially tau equivalent halves instead of parallel) means the GF split half is not inflated. That is, GF < SB. They are only equal when the variances of the halves are equal. Again, having essentially tau equivalent halves is somethimes easier to justify than essentially equivalent items.
>
>I'll be away from work for the next eight days, back on Friday 28 April.
>
>Best wishes, thanks for the discussion,
>Paul Gardner
>
>Attachment Converted: "d:\paul\email\attach\Paul.Gardner2.vcf"
>
------------------------------------
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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