nbNA <- function(v) { return(sum(is.na(v))) } # nombre de NA dans v

print( nbNA( c(1,3,8,NA,2,NA,5) ) )

library(gdata)
age <- read.xls("essai.xls")$AGE

print( nbNA( age ))

data <- read.xls("essai.xls")

print( apply(X=data,F=nbNA,M=2) )