[R] (Newbie) Aggregate for NA values

[Vivek Satsangi]

Folks,

Sorry if this question has been answered before or is obvious (or
worse, statistically "bad"). I don't understand what was said in one
of the search results that seems somewhat related.

I use aggregate to get a quick summary of the data. Part of what I am
looking for in the summary is, how much influence might the NA's have
had, if they were included, and is excluding them from the means
causing some sort of bias. So I want the summary stat for the NA's
also.

Here is a simple example session (edited to remove the typos I made,
comments added later):

> tmp_a <- 1:10
> tmp_b <- rep(1:5,2)
> tmp_c <- rep(1:2,5)
> tmp_d <- c(1,1,1,2,2,2,3,3,3,4)
> tmp_df <- data.frame(tmp_a,tmp_b,tmp_c,tmp_d);
> tmp_df$tmp_c[9:10] <- NA ;
> tmp_df
   tmp_a tmp_b tmp_c tmp_d
1      1     1     1     1
2      2     2     2     1
3      3     3     1     1
4      4     4     2     2
5      5     5     1     2
6      6     1     2     2
7      7     2     1     3
8      8     3     2     3
9      9     4    NA     3
10    10     5    NA     4
> aggregate(tmp_df$tmp_d,by=list(tmp_df$tmp_b,tmp_df$tmp_c),mean);
  Group.1 Group.2 x
1       1       1 1
2       2       1 3
3       3       1 1
4       5       1 2
5       1       2 2
6       2       2 1
7       3       2 3
8       4       2 2
# Only one row for each (tmp_b, tmp_c) combination, NA's getting dropped.

> aggregate(tmp_df$tmp_d,by=list(tmp_df$tmp_c),mean);
  Group.1    x
1       1 1.75
2       2 2.00

What I want in this last aggregate is, a mean for the values in tmp_d
that correspond to the tmp_c values of NA. Similarly, perhaps there is
a way to make the second last call to aggregate return the values of
tmp_d for the NA values of tmp_c also.

How can I achieve this?

--
-- Vivek Satsangi
Student, Rochester, NY USA