Talk:Cronbach's alpha

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It is very difficult to find information on the appropriateness of Cronbach's alpha for continuous variables. Can someone please add a statement about using Cronbach's alpha for the reliability of discrete vs. continuous variables?

Do you mean here or in the article? I'm a specialist in Cronbach's alpha, and there is no problem whatsoever with continuous variables. JulesEllis 22:56, 14 January 2007 (UTC)

It would be nice to have a guide as to what are considered adequate values for Cronbach alpha, what the implications are for using a test with a Cronbach alpha of, say .5 Tim bates 11:08, 9 October 2006 (UTC)

Some people say that 0.70 is a professional standard for reliability. I'm not aware of where this comes from. I've also heard 0.60.

However. you cannot blindly apply a simple rule of thumb for three reasons. First, alpha assumes that the components are multually parallel (or, at least, multually tau-equivalent). This is rare, if it exists at all. In the typical situation the components are indvidual items which diffier in content and difficulty. In these cases, alpha is not an unbiased estimate of reliabiity, but instead is a lower bound on reliability. So, if you have a test with alpha = 0.50 but the test is composed of hertogeneous items, the reliability may well be much higher than 0.50.

Second, reliability is sample-specific. An alpha of 0.50 in a homogeneous subsample (with reduced true score variability) may be quite high.

And third, the reliability of a test (especially the internal consistency estimates of reliability) cannot be evaluated without respect to the purpose of an instrument. If a test is included in a screening battery, a reliability of 0.50 might be sufficient (because it will be combined with other scores). Or if the test is designed to be pass/fail, the overall reliability may be low (like 0.50) while the pass/fail consistency is quite a bit higher. Amead 01:07, 11 October 2006 (UTC)

The 'alpha' in this title is a really bad idea. Any way to change it (Move has just failed for me)?

Charles Matthews 13:59, 15 Feb 2004 (UTC)

How about a redirect, so this title would still be in the database (for those whose software is happy with it), and the content would be at Cronbach's alpha. Vicki Rosenzweig 14:23, 15 Feb 2004 (UTC)

OK, tried that with copy+paste and as you can see it goes to 'Cronbach's α'. So, I'm not up to speed with the coding for the alpha.

Charles Matthews 14:33, 15 Feb 2004 (UTC)

Cronbach's α or Cronbach's &_alpha; (without the underscore is contrary to the ASCII norm of the English Wikipedia headings, so this is now Cronbach's alpha --Henrygb 00:36, 10 Aug 2004 (UTC)

I realise it's what comes through from a template, but it seems to me misleading to say that the title "Cronbach's alpha" is "wrong". You'll see it written in that form in innumerable articles that use coefficient alpha - and Cronbach himself spelt it out in his original article title. Any objections to just deleting the template? seglea 5 July 2005 23:29 (UTC)

[edit] Condition under which alpha is the reliability

It was stated that alpha is equal to the reliability if the items are parallel, and smaller otherwise. This is incorrect. The necessary and sufficient condition for alpha to be equal to the reliability is that the items are essentially tau-equivalent (Lord & Novick, 1968). This allows the items to have different means and even different variances. I know that most text books deal only with the parallel case, but then the statement should be "if" and not "if and only if". So I corrected this.

JulesEllis 23:04, 14 January 2007 (UTC)

[edit] Other roles of alpha

The current article was only about the role of alpha in classical test theory. This is unfortunate, because Cronbach himself rejected much of that theory and developed the generalizability theory for this reason. I also added a little section about the intra-class correlation and factor analysis. I think I should write an article about that too, because it happens too often that people are not aware that they are actually the same in many two-facetted applications. JulesEllis 06:04, 15 January 2007 (UTC)