subset columns with common values in long data frame

Question

I have the following data frame:

Group 1 ID A    Value
Group 1 ID B    Value
Group 1 ID C    Value
Group 2 ID B    Value
Group 2 ID C    Value
Group 3 ID B    Value
…   …   …

I am trying to use dplyr to get the mean value for each of the same ID across groups (eg the mean of the value of ID B across group 1, group 2, and group 3). However, not every group has all of the IDs so I wanted to subset so that only means for IDs which are in all groups get computed. I know that I can group_by(dataFrame, group) %>% filter subset %>% group_by(id) %>% mutate(mean) but I don't know what code to place in the filter subset.

Answer 1

How about

df %>%
  group_by(id) %>%
  mutate(count  = n()) %>%
  filter(count != ngroups) %>% #...

So basically remove all the rows in the dataframe that correspond to an ID that doesn't appear in all groups, then perform the computation.