[R] Code for Coefficent (Cronbach's) Alpha

[Ben Bolker]

  Caveat, I haven't spent a lot of time thinking about this and you should
check that this really does what you want!  But ...

  I think you can do this more compactly (and faster) using apply().

  score.total <- apply(x,1,sum)
  var.items <- sum(apply(x,2,var))
  z.score.total <- apply(z.score.total,1,sum)
  z.var.items <- sum(apply(z.x,1,sum)

  Doing it this way you can dispense with the most of the setting-to-zero
stuff.

  -- assuming that you want score.total and z.score.total to be vectors
containing the row sums and var.items and z.var.items to be the sum of the
variances of the columns.

 You can of course condense things even more, for example

  var.total <- var(apply(x,1,sum))
  z.var.total <- var(apply(z.score.total,1,sum))

 You have to decide on your own about the tradeoff between
compactness/efficiency and legibility.

On Mon, 17 Jul 2000, Magill, Brett wrote:

> reliability.alpha<-function(x){
> 
> 	score.total<-0
> 	var.items<-0
> 	var.total<-0
> 	z.x<-0
> 	z.score.total<-0
> 	z.var.items<-0
> 	z.x<-scale(x, center = TRUE, scale = TRUE)
> 	n.items<-length(x)
> 
> 	for (i in 1:n.items) score.total<-score.total + x[,i]
> 	for (i in 1:n.items) var.items<-var.items + var(x[,i])
> 	for (i in 1:n.items) z.score.total<-z.score.total + z.x[,i]
> 	for (i in 1:n.items) z.var.items<-z.var.items + var(z.x[,i])
> 
> 	var.total<-var(score.total)
> 	z.var.total<-var(z.score.total)
> 
> 	cronbachs.alpha<-(n.items/(n.items-1))*((var.total -
> var.items)/var.total)
> 	z.cronbachs.alpha<-(n.items/(n.items-1))*((z.var.total -
> z.var.items)/z.var.total)
> 
> 	return(cronbachs.alpha, z.cronbachs.alpha)}




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-- 
Ben Bolker                                  bolker at zoo.ufl.edu
Zoology Department, University of Florida   http://www.zoo.ufl.edu/bolker
318 Carr Hall/Box 118525                    tel: (352) 392-5697
Gainesville, FL 32611-8525                  fax: (352) 392-3704

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