On 12/07/2011 12:26 PM, Doran, Harold wrote:
This is just posed out of curiosity, (not as a criticism per se). But what is 
the functional role of the argument na.rm inside the mean() function? If there 
are missing values, mean() will always return an NA as in the example below. 
But, is there ever a purpose in computing a mean only to receive NA as a result?

The general idea in R is that NA stands for "unknown". If some of the values in a vector are unknown, then the mean of the vector is also unknown. NA is also used in other ways sometimes; then it makes sense to remove it and compute the mean of the other values.

Duncan Murdoch

In 10 years of using R, I have always used mean() in order to get a result, 
which is the opposite of its default behavior (when there are NAs). Can anyone 
suggest a reason why it is in fact desired to get NA as a result of computing 
mean()?

>  x<- rnorm(100)
>  x[1]<- NA

>  mean(x)
[1] NA

>  mean(x, na.rm=TRUE)
[1] 0.08136736

If the reason is to alert the user that the vector has missing values, I 
suppose I could buy that. But, I think other checks are better

Harold


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