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
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