Subset cases in which there were more than 3 observations in longitudinal data?

Question

Have a set of longitudinal data in which measures were repeatedly collected at various waves (see example of set up below. As this sort of data goes however, there was attrition, with some waves stopping before the study ended. However, my analysis has the assumption that each participant have at least 3 observations

ID	Wave	Score
1000	0	5
1000	1	4
1001	0	6
1001	1	6
1001	2	7

How would I subset only those IDs (subjects) that have at least 3 observations? I've looked into similar questions on stackoverflow but they do not seem to fit this specific issue.

Answer 1

Using base R, you could try this one-liner.

out <- with(df, df[ID %in% names(which(sapply(split(df, ID), nrow) > 2)), ])

Output

> out
    ID Wave Score
3 1001    0     6
4 1001    1     6
5 1001    2     7

Data

df <- data.frame(
  ID = unlist(mapply(rep, 1000:1001, 2:3)),
  Wave = c(0,1,0,1,2),
  Score = c(5,4,6,6,7)
)