`sorenson` <-
function(x,y) 
{
  ai <- (y[y>0]-x[y>0])!=y[y>0]
  a <- length(ai[ai==TRUE])
  b<-length(x[x>0])-a
  c<-length(y[y>0])-a
  sor<-2*a/(2*a+b+c)
  return(sor)
}