Hi Brian. Re treating the sum of the eight items as interval, here's an
interesting (and provocative) article I was alerted to just yesterday:
Liddell TM, Kruschke JK. Analyzing ordinal data with metric models: What
could possibly go wrong?. Journal of Experimental Social Psychology. 2018
Nov 1;79:328-48.
https://osf.io/9h3et/download?format=pdf
If you have institutional access to sciencedirect.com, you can get the final
published article here:
https://www.sciencedirect.com/science/article/pii/S0022103117307746
From p. 344 in the final published article:
"Some authors have argued that, despite the ordinal character of individual
Likert items, averaged ordinal items can have an emergent property of an
interval scale and so it is appropriate to apply metric methods to the
averaged values (e.g., Carifio and Perla, 2007, Carifio and Perla, 2008. It
is intuitively plausible that the averaging could produce data that at least
look more continuous and therefore may not suffer from the problems pointed
out above. Unfortunately that intuition is wrong. We show in this section
that an average of ordinal items has the same problems as a single item."
HTH.
bdates wrote
> I have been asked to analyze data from a large (n=402) study of an
> intervention for adolescents. One of the measures, that is quite highly
> used, is the CAFAS, which assesses functioning in children and youth, age
> 6 to 18. There are eight items, scored from 0 to 30 in increments of 10
> (i.e., 0, 10, 20, 30). The eight items produce a total score ranging from
> 0 to 240. A review of the literature does not reveal really good
> reliability, either alpha, or ICC (the measure is clinician-completed).
> Item-total correlations range from .20 to .57. The distribution of scores
> on all items is skewed, either positively or negatively. I'm proposing to
> analyze each of the scales as if they were ordinal and the total score as
> interval, using a repeated measures approach since there are multiple
> measures per youth. I neither designed the study nor chose the measures.
> I've simply been commissioned to analyze the data. Any thoughts on
> treating the items as ordinal? I know it's conservative, but I have
> difficulty with a four-point discontinuous scale as really interval in
> nature.
>
>
> Brian
>
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