select last observation from longitudinal data

Question

I have a data set with several time assessments for each participant. I want to select the last assessment for each participant. My dataset looks like this:

Answer 1

If you're just looking for the last observation per person ID, then a simple two line code should do it. I am up always for simple base solution when possible while it is always great to have more than one ways to solve a problem.

dat[order(dat$ID,dat$Week),]  # Sort by ID and week
dat[!duplicated(dat$ID, fromLast=T),] # Keep last observation per ID

   ID Week Outcome
3   1    6      42
8   4   12      85
13  9   12      84