Paul R Swank wrote: > > I disagree with the statement that the split-half reliability coefficient > is of no use anymore. Coefficient alpha, while being an excellent estimator > of reliability, does have one rather stringent requirement. The items must > be homogeneous. This is not always the case with many kinds of scales, nor > should it be. In many cases homogeneity of item content may lead to reduced > validity if the consruct is too narrowly defined. Screening measures often > have this problem. They need to be short but they also need to be broad in > scope. Internal consistency for such scales would suffer but a split half > procedure, which is much less sensitive to item homogeneity, would fit the > bill nicely. I have four responses to this: 1. Split-half requires the items to be divided into two "equal" halves. How is this to be done? Odd/even? First half/second half? Randomly? Cronbach's alpha does not depend on this arbitrary division into halves. 2. Stanley and Hopkins (1972) demonstrated that Cronbach's alpha was essentially equivalent to the "mean of all possible split-half reliability estimates". DeVellis (1991) demonmstrates that if the items in a scale have similar variances (a condition frequently met in well-designed scales), it can be shown that the value of alpha (called standardised alpha) is algebraically equivalent to the Spearman-Brown formula for estimating split-half. In other words, there is no great difference conceptually between the two. 3. Many writers use the term 'homogeneity' to bolster arguments in discussions of reliability and validity. In a paper I have completed recently which is currently under review for publication, I show that the term has about six different meanings in the literature. Whenever I read the word now, I respond, What exactly does the writer mean by homogeneity here? 4. If, by homogeneity, you mean all the items are measuring a similar construct, i.e. the item scores all inter-correlate with each other because they are indicators of a unidimensional construct, then the assertion that Cronbach's alpha depends on being this being the case is demonstrably untrue. Cronbach's alpha will be high as long as every item in a scale correlates well with at least some other items, but not necessarily all of them. Homogeneity is not a "stringent requirement" for a high Cronbach alpha level at all. Cronbach's alpha is simply a measure of reliability; it is not an indicator of unidimensionality, a point widely misunderstood in the literature. Paul Gardner
begin:vcard n:Gardner;Dr Paul tel;cell:0412 275 623 tel;fax:Int + 61 3 9905 2779 (Faculty office) tel;home:Int + 61 3 9578 4724 tel;work:Int + 61 3 9905 2854 x-mozilla-html:FALSE adr:;;;;;; version:2.1 email;internet:[EMAIL PROTECTED] x-mozilla-cpt:;-29488 fn:Dr Paul Gardner, Reader in Education and Director, Research Degrees, Faculty of Education, Monash University, Vic. Australia 3800 end:vcard
