> 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.
It doesn't offer guidance up front exactly, no, but it provides feedback on
whether items work, and an important (imv) conceptual framework for test
construction. For example, if you have the Rasch model in mind, you look to
developing items of a range of difficutly (or 'affective intensity'). You
wouldn't necessarily think to do this if you were only using other
techniques, yet it is surely important. Take the extreme example of a test
in which every item is of the same affective intensity -- say for
'satisfaction with your bank'. Everyone who is higher than a certain
satisfaction level would be expected to agree with all items, whereas
everyone below a certain satisfaction would be expected to disagree (of
course, this probably won't happen but it in reality you may get something
approaching this situation). In this case, the instrument will not
effectively discriminate between a person somewhat lower on satisfaction than
all your items are targeted toward (eg lower than the level of satisfaction
needed to just agree with a certain statement), versus someone far less
satisfied than that again (both will simply disagree with all or most
statements). Conversely, if you have a set of items which target a range of
satisfaction levels, you expect different scores for most people on the test,
dependent upon their particular level of satisfaction (roughly in keeping
with a Guttman structure). Surely that's what you should be after! It also
tells you whether categories on Likert scales function well or not. For
example, 'neutral' categories don't typically work very well (mind you, I
can't give empirical evidence, this is just what people have found in
experience, including myself). Looking at which items fit and which don't
obviously provides critical information about the nature of the construct
itself. Via feedback from these sorts of things, you certainly get an idea
of what kinds of items and response modes effectively elicit responses
indicative of a latent trait. That is, responses governed stochastically by
item 'difficulty' and person 'ability' (or affective intensity and
satisfaction).
> 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.
>
Sure, this may be the case. Not necessarily though. I have tried the same
on a couple of occasions and found that the decisions were quite different
based on Rasch analysis vs Factor Analysis. This would in fact have been
expected given the Rasch analysis because the Likert categories did not
effectively discriminate with respect to the latent trait. Correlational
techniques obviously rely upon having roughly equal intervals between score
points.
>
> 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
Rasch measurement is in essence equivalent to Thurstone's law of comparative
judgement except that (a) the person parameter is substituted for one of the
item parameters, and (b) the logistic function is substituted for the
normal. It is based on the same logic but the above trick allows separation
of person and item parameters. Yes, all are methods with attempt to measure
attitudes (or whatever) by producing a score. However, Rasch uses a
non-linear 'transformation' of the raw score so is fundamentally different
from Likert's approach (not Thurstone's of course, in that respect). On your
last statment above, many would argue that these psychometric methods are
obsolete precisely because they were replaced by a better model (based on
requirements of fundamental measurement).
Steve.
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