Re: st: RE: Mean test in a Likert Scale

[Nick Cox <njcoxstata@gmail.com>]

I bow to others' expertise and experience on the minutiae here, some
of which seem almost theological in character. For "likert" read
"Likert", passim.

My impression from the thread, however, is that some seem to think
that extreme views vs moderate views are the issue, and some seem to
think that agree vs disagree is the issue. I can't detect a consistent
position among posters about how intermediate points are to be handled
either.

This disagreement to me adds flavour to the wording "arbitrary".

Naturally, I am interested to learn that dichotomising a Likert scale
is something that researchers think is sometimes justifiable, but I
ever met it I would expect some discussion of quite how it was done
and why that made sense.

Nick

On Fri, Aug 31, 2012 at 5:32 PM, David Radwin <dradwin@mprinc.com> wrote:
> Rob,
>
> It may be the case that not labeling the middle points of a scale, as in
> your first example, justifies the assumption of equal spacing (deltas).
> But the literature suggests that verbally labeling all points on a scale,
> as in your second example, leads to more reliable measurement. See, for
> example:
>
> Alwin DF, Krosnick JA. 1991. The reliability of survey attitude
> measurement: The influence of question and respondent attributes. Sociol.
> Methods Res. 20:139-81.
> http://deepblue.lib.umich.edu/bitstream/2027.42/68969/2/10.1177_0049124191
> 020001005.pdf
>
Rob Ploutz-Snyder

>> My 2 cents...when designing these sorts of instruments...
>>
>> I was trained that a true likert scale doesn't label each of the
>> points in the 5-point (or other) scale, but instead has only TWO
>> labels at each extreme.  For example:
>>
>> I like Statalist..............      Completely Disagree   1  2  3  4
>> 5    Completely Agree
>>
>> This is in CONTRAST to a scale that would label each and every point
>> (sometimes called "likert-type" or "modified-likert") for example:
>>
>> 1=completely disagree
>> 2=disagree
>> 3=neutral
>> 4=agree
>> 5=completely agree
>>
>> With true likert scales, while still not continuous in scale, the
>> distance between each category in a true likert scale is not
>> subjective.  The delta between "1" and "2" is the same as the delta
>> between "2" and "3" etc.  and it is assumed that survey respondents
>> can appreciate this.  The same cannot be assumed about the difference
>> between "completely disagree" and "disagree" being equal to the delta
>> between "disagree" and "neutral."
>>
>> So in that way, a  true-likert scale removes some of the subjectivity
>> on the deltas and seems to achieve a more proper ordinal scale as
>> opposed to purely categorical.
>>
>> Still doesn't justify using parametric statistical techniques...
>> However, most well-vetted Sociology or Psychological instruments are
>> designed to use multiple questions that, together, are used to measure
>> a particular construct.  Social scientists don't usually intend to
>> compare responses on single questions, but instead ask many questions
>> that cluster together, often verified by exploratory or confirmatory
>> factor analysis, where "factor scores" are then created to capture the
>> overall construct of interest.  These factor scores can be derived by
>> different methods, the simplest being a mean of the items that cluster
>> together, but usually by more sophisticated regression-based methods
>> that weigh each item according to how well it correlates with the
>> overall factor structure.  These factor scores are continuously
>> scaled, unlike the individual items that were used to derive them, and
>> it is these factor scores that are often analyzed by various
>> parametric statistical techniques.
>>
>> Whether or not the factor scores  are normally distributed in the
>> population (the real question) is dependent on the particulars of each
>> research study, but I don't categorically deny that the assumption is
>> invalid.
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