[EMAIL PROTECTED] wrote: > > In a way, yes, they were superseded. There is a school of thought now > in which the proponents would argue (imv successfully) that the > approach you�ve outlined culminated in Rasch�s Simple Logistic Model. > Over and above the benefits of the Thurstone�s comparative judgements, > the Rasch model allows you to place person parameters and item > parameters on the one metric, and to eliminate person parameters in the > estimation of item parameters, and vice versa. Most importantly, > according to proponents, the model allows you to achieve the > requirements of fundamental measurement (including conjoint additivity > and invariance --I can give some explanation and/or quotes if you like) > provided a reasonable fit of data to the model. > > The Rasch model is a stochastic one in which responses are said to be > governed only by a person parameter and an item parameter. While > Thurstone used the normal curve in his law of comparative judgement, > Rasch used the logistic approximation (very close of course). Tthis > allowed him to separate person and item parameters � a very significant > achievement imv. In turn, he called the outcome �specific > objectivity�. That is, the estimates of item parameters are > independent of the particular set of persons used to derive them, and > the estimates of person parameters are independent of the particular > set of items used to derive them (this is algebraic fact under the > model, then the point is � do the data fit the model?). Such > objectivity is key in the physical sciences (I have a quote in which > Andrich (see below) shows how this situation applies for a = f/m, > whereby a comparison of accelerations is independent of the force that > is instrumental in causing them). > > There are various sources of information on this. Try www.rasch.org/ > for some discussion of the properties of the Rasch model, applications, > and various other things. Or there is �Rasch Models for Measurement� > by David Andrich. But there are various refs on the above website. If > you want to know anything more, just ask, and I�ll help if I can � > though I�m relatively new to Rasch myself (I�ve done courses with David > Andrich, who developed the �Extended Logistic Model� for use with > Likert scale data rather than dichotomous data, and himself trained > with Rasch for a time). > This is all fine, but please remember that Rasch is essentially a sophisticated (and much more thoughtful) mathematical model for describing the properties of items and people; it offers no guidance on how to write items for an attitude measurement scale. One still has to define constructs, write items and design an appropriate response mode. Rasch provides a mathematical rationale for selecting items for inclusion in a scale, using the criterion of "fit to the model". I don't claim great expertise here, but when I ran an attitude scale through a Rasch analysis and a traditional item analysis/factor analysis (many years ago), the decisions reached about which items to include or exclude were not too different. I regard Rasch as a synthesis of the Thurstone and Likert techniques. Thurstone placed much emphasis on item calibration, getting large numbers of judges to rate where items were located on a supposedly interval scale, but used only a small number of items to measure individuals' attitudes. Likert placed much emphasis on person measurement, using a large number of items to measure people's attitudes, but placed less emphasis on the calibration of item properties. Rasch places equal emphasis on person measurement and item calibration, and uses a common measurement scale for both. However, bear in mind that all are psychometric methods which attempt to measure attitudes by producing a scale score. I took the original question that started off this thread to wonder about psychometric methods were obsolete, and not whether Likert and Thurstone had been replaced by better mathematical models. 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
