Dear r-help reader,

I have some problems with the aggregate function.

My datframe looks like
 >frame

   Day Time V1 V2
1   M    0  3 NA
2   M    0  4 NA
3   M    0  5  2
4   M    1 NA  4
5   M    1 10  6
6   T    0  4 45
7   T    1  4  3
8   T    1  3  2
9   T    1  6  1

I used the aggegate function to obtain the mean in V1 and V2 over the 
grouping variable
Time and Day

  aggregate(frame[,c(-1)],list(frame$Day,frame$Time),mean)
   Group.1 Group.2 Time       V1 V2
1       M       0    0 4.000000 NA
2       T       0    0 4.000000 45
3       M       1    1       NA  5
4       T       1    1 4.333333  2
 >

My problem is now that I do not obtain a 'mean' for Day=M/Time=0 and 
Day=M/Time=1,

because aggregate ignores all values for a grouping variable if NA 
occurs.

I'm now hoping for some help so that the mean is still calculated in 
this group.

My table should look like:

  aggregate(frame[,c(-1)],list(frame$Day,frame$Time),mean)
   Group.1 Group.2 Time       V1 V2
1       M       0    0 4.000000 2
2       T       0    0 4.000000 45
3       M       1    1       10  5
4       T       1    1 4.333333  2

 >

I hope my description makes sense and appreciate any help.

Yours
Frank

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