The difference of na.rm and na.omit in R

Question

I\'ve just started with R and I\'ve executed these statements:

library(datasets)
head(airquality)
s <- split(airquality,airquality$Month)
sapply(s, function(x         


        

Answer 1

They are not supposed to give the same result. Consider this example:

exdf<-data.frame(a=c(1,NA,5),b=c(3,2,2))
#   a b
#1  1 3
#2 NA 2
#3  5 2
colMeans(exdf,na.rm=TRUE)
#       a        b 
#3.000000 2.333333
colMeans(na.omit(exdf))
#  a   b 
#3.0 2.5

Why is this? In the first case, the mean of column b is calculated through (3+2+2)/3. In the second case, the second row is removed in its entirety (also the value of b which is not-NA and therefore considered in the first case) by na.omit and so the b mean is just (3+2)/2.