isoMDS and stress

Hi all,

I try to calculate the stress of a configuration using the formula in
the isoMDS help, but I don't have the same result than the stress
returned by isoMDS :

library(mva)
library(MASS)
data(swiss)
swiss.x <- as.matrix(swiss[,-1])
swiss.dist <- dist(swiss.x)
swiss.mds <- isoMDS(swiss.dist)

initial  value 2.979731 
iter   5 value 2.431486
iter  10 value 2.343353
final  value 2.338839 
Converged

delta.x <- swiss.mds$points
delta.dist <- dist(delta.x)

stress <- function(avant, apres) {

+ 	l <- length(avant)
+ 	num <- 0
+ 	denum <- 0
+ 
+ 	for (i in 1:l) {
+ 		num <- num + (avant[i] - apres[i])^2
+ 		denum <- denum + apres[i]
+ 	}
+ 
+ 	sqrt(num / denum)
+ }

swiss.mds$stress

[1] 2.338839

stress(swiss.dist, delta.dist)

[1] 0.4196349

Any idea ?
Thanks

--
Stephane