[R] outliers for Likert scale data

[Helios de Rosario]

>>> El día 01/09/2013 a las 15:13, Helen Sawaya
<helensawaya at hotmail.com>
escribió:
> Dear R experts,
> I have data from a questionnaire that I would like to factor analyse.
It is 
> in a likert scale form (0-3). I would like to check first for
univariate and 
> multivariate outliers but the most common ways of doing so assume the
data is 
> continuous and normal- neither of which is the case here. I found an
article 
> discussing this (Outlier Detection in Test and Questionnaire Data by
Wobbe P. 
> Zijlstra, L. Andries van der Ark, and Klaas Sijtsma), but I was
wondering if 
> I could get the exact R code on how to implement the outlier
detection 
> analyses.

I have not found an exact implementation of that article, but one of
its authors (van den Ark) has published the "mokken" package with some
methods referred to in it:
https://sites.google.com/a/tilburguniversity.edu/avdrark/mokken

The ESD method for identifying outliers, also used in the paper to
handle outlier scores, is implemented (together with others) in the
package "parody":
http://www.bioconductor.org/packages/release/bioc/html/parody.html

Hope it helps
Helios De Rosario


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