reliability, scale scores in the psych package
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