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)
+ }