When processing data it is common to test if an observation belongs to a set. Let’s suppose that we want to see if the sample code belongs to a set that includes A, B, C and D. In R it is easy to write something like:

```
inside.set <- subset(my.data, code %in% c('A', 'B', 'C', 'D'))
```

Now, what happens if what we want are the observations that are not in that set? Simple, we use the negation operator (!) as in:

outside.set <- subset(my.data, !(code %in% c('A', 'B', 'C', 'D')))

In summary, surround the condition by !().