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comparing attitudes of 2 groups / likert scales?

7 messages · Mona_m, Tal Galili, David Winsemius +2 more

Original
Mona_m
# 
Hi,

I have just found this forum, and it looks like a great place to get some
help (I hope)
For my dissertation, which is due way too soon, I am doing a survey,
comparing attitudes of 2 independent groups, with 5 scale likert questions.
Basically I want to show if they have similar or different attitudes. I am
testing 4 hypotheses, and have in total about 20 questions. 

I have to say my statistic skills are very basic and very rusty, we had some
lectures two years ago, where we were introduced to R. I looked through my
notes, and back then we did a one sample t-test to analyse likert type
questions. I believe I would need to do a 2 sample unpaired t-test.  It
would be great if someone could give me some feedback if this test is the
most suitable one for my purpose, and maybe could explain to me what?s the
easiest way to do this in R?

You would help me loads!! 
Many thanks in advance
Mona
Dieter Menne
#
Mona_m wrote:
Using an unpaired (in you case) t-test on Likert scale is a bit risky,
because the Gaussian distribution might be severely violated. It might be Ok
if your data are reasonable centered around "moderate", but frequently we
have responses where all but one subject replied with "very good". If you
can create a sum of scores, these are frequently more suitable for being
analyzed by some quasi-continuous method, and using a non-parametric
Wilcoxon test might avoid reviewer comments in some areas of research.

If you only have few levels, something like polr (in MASS) and the plots
created from it by Fox/Anderson might be an alternative (google for
Fox/polytomous effects). These results are more difficult to interpret and I
have seen cases where papers using this where rejected in medical journals
(why don't you use Wilcoxon?).

Overall, when you have more complex cross-over designs with additional
crossed variables, violating Gaussian assumptions for me seems to be the
lesser evil compared to violating independence. "Assess independence, equal
variance and normality -in that order" (van Bell, Statistical rules of
thumb). I remember Douglas Bates mumbling something along the same lines,
but he mentioned a 10 level scale.

Dieter
David Winsemius
#
On Apr 19, 2010, at 10:08 AM, Tal Galili wrote:

            

      
      
      
    

        
If you are thinking of using that quote, you might want to check the  
spelling of his name. My memory is van Belle.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Dieter Menne
    

    

      
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David Winsemius wrote:

            

      
      
      
    

        
Sorry, I thought I had corrected that before mailing.

@BOOK{vanBelle2002,
  title = {Statistical rules of thumb},
  publisher = {Wiley series in probability and statistics},
  year = {2002},
  author = {Gerald van Belle}
}

  
    

      
      
    

  


  


      

    

    

      
      

        
      

    

    

      
      

        


  

        

      Peter Ehlers
    

    

      
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On 2010-04-19 9:15, Dieter Menne wrote:

            

      
      
      
    

        
And a very fine book it is, too. Highly recommended.