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rsubset

Remove groups with less than three unique observations

I would like to subset my data frame to keep only groups that have 3 or more observations on DIFFERENT days. I want to get rid of groups that have less than 3 observations, or the observations they have are not from 3 different days.

Here is a sample data set:

Group   Day
1       1 
1       3
1       5
1       5
2       2
2       2  
2       4 
2       4
3       1
3       2
3       3
4       1
4       5

So for the above example, group 1 and group 3 will be kept and group 2 and 4 will be removed from the data frame.

I hope this makes sense, I imagine the solution will be quite simple but I can't work it out (I'm quite new to R and not very fast at coming up with solutions to things like this). I thought maybe the diff function could come in handy but didn't get much further.

Solution

  • With you could do:

    library(data.table)
DT[, if(uniqueN(Day) >= 3) .SD, by = Group]

    which gives:

       Group Day
1:     1   1
2:     1   3
3:     1   5
4:     1   5
5:     3   1
6:     3   2
7:     3   3

    Or with dplyr:

    library(dplyr)
DT %>% 
  group_by(Group) %>% 
  filter(n_distinct(Day) >= 3)

    which gives the same result.