[R] reliability, scale scores in the psych package

[William Revelle]

Ista,
   As you figured out, psych reverses items by subtracting from the 
maximimum + minimum possible for each item.  (i.e., for  items going 
from 1 to 4, it reverses items by subtracting from 5).

If all of the items have the same potential range  then you can just 
let it figure out the range by itself.  If they differ in their 
ranges (some items are 0 - 1 items, some are 1-9 items, etc., then 
you need to give it the maximum and minimum vectors to use.

The min and max are figured out from all the items used in an 
inventory, rather than just the items used in a particular scale. 
This makes particular sense when you are scoring multiple scales from 
the same inventory.

In answer to your first question (what packages do I tend to use for 
scale construction?), the answer is that I tend to use the psych 
package for basic analysis, and then the sem package for structural 
equation analysis.

Bill





At 10:45 AM -0400 3/10/09, Ista Zahn wrote:
><snip>
>>  Second question: I spent some time with the psych package trying to
>>  figure out how to use the score.items() function, and it's become
>>  clear to me that I don't understand what it's doing. I assumed that
>>  setting a key equal to -1 would result in the item being reverse
>>  scored, but I get weird results, as shown below. When I try to reverse
>>  score (by setting a value of -1 in the key), I get scale scores that
>>  don't add up (e.g., the mean score is reported as being larger than
>>  the maximum item score). How is the score.items() function intended to
>>  be used? Do I need to reverse score items before using score.items()?
>
>I did it again--it seems like I always figure out the answer just
>after I ask for help. The score.items() function needs to know the
>maximum of the scale in order to reverse score. For some reason, the
>maximum appears to be calculated from all the scores, not just scores
>that have a 1 or a -1 in the key. On a hunch I set the max argument to
>a vector of scale maxima, and it worked. I'm still interested in
>responses to question 1 though.
>
>Thanks again,
>Ista
>
><snip>


-- 
William Revelle		http://personality-project.org/revelle.html
Professor			http://personality-project.org/personality.html
Department of Psychology             http://www.wcas.northwestern.edu/psych/
Northwestern University	http://www.northwestern.edu/
Use R for psychology                       http://personality-project.org/r